Tom Coughlin is in his 11th year as a professional head coach, and he has won exactly 11 games more than he has lost. That’s as close to mediocrity as you’re allowed to get in the National Football League. If you were more mediocre than that, you’d be an excoach.

The truly remarkable thing about Coughlin’s career is not his mediocrity. Mediocrity implies some kind of consistency. What is remarkable about Coughlin is the wild up and down swings his teams have taken to achieve a 94–85 (including postseason) record. Swings from season to season, game to game, even quarter to quarter.

In 1995, Coughlin took over the reins of the expansion Jacksonville Jaguars, and, as you’d expect, didn’t do much with them, finishing 4–12. In their second season, the Jags showed a remarkable improvement, jumping to 9–7; it was the only year any Coughlin team has won two playoff games. 1997 and 1998 saw Jacksonville go 11–5; in 1999, Coughlin had his best team ever, winning 14 of 16 regular season games, finishing sixth in points scored on offense, and first in fewest points allowed. In the first round of the playoffs, the Jags humiliated Miami 62–7. In the AFC Championship game the next week, they were humiliated 14–33 by a Tennessee team that’s was no better than they were.

Coughlin’s stay at Jacksonville was all downhill from there; over the next three seasons, the Jags were a combined 19–29 and failed to make the playoffs.

Coughlin’s three seasons with the Giants have been an accelerated version of his tenure with Jacksonville: slow start, immediate improvement, swift decline. The Giants were 6–10 in 2004, rebounded to 11–5 last year, and, currently at 6–5, seem to be on the verge of disintegrating. Like the Jacksonville years, Coughlin’s three seasons in New York have produced some baffling defeats. Here are a few of the most inexplicable:

• On November 6, 2005, the Giants won an impressive 24–6 victory over the 49ers at San Francisco. The next week, with a 6–2 record, they were expected to dominate the 3–5 Minnesota Vikings at the Meadowlands on both sides of the ball. They did, outgaining Minnesota in yards (405–137), first downs (25–11), and in time of possession (35:35 to 24:25). The Vikings gained exactly 12 yards on the ground for an average of 0.6 a carry. Still, without an offense and very little defense, Minnesota won the game 24–21.

How did the Vikes do it? First, by shifting coverages against Eli Manning. “We baited him into calling certain plays and then moved our defensive people around,” said Vikings coach Mike Tice, who was polite enough not to say that Manning and Coughlin never caught on. The Vikings intercepted four Manning passes — three by Darren Sharper, who returned one for a touchdown. But that was far from all, the Giants did much, much more to lose the game: Minnesota had an incredible 394 return yards, returning both a punt and a kickoff for a touchdown. It was the first time in the history of the NFL that a team scored touchdowns on an interception, punt, and kick return in the same game.

• Two weeks later, the Giants lost another game 24–21; this time in Seattle in a performance entirely different but every bit as bizarre. This one couldn’t be put off to Manning, who was brilliant, shredding the Seahawks defense for 324 yards while Tiki Barber racked up 166 more on the ground. In all, New York outgained the Seahawks by 135 yards, and again found a way to lose. Sixteen penalties — their most in the 56 years of the franchise — negated more than 100 yards of offense and brought back what would have been the winning touchdown. An amazing 11 of the flags were for false starts. The Giants blew three chances to win when Jay Feely, who had kicked 23 of 25 field goals up to that point in the season, went wide left, short, and wide right on fourth quarter and overtime field goal tries. Feely stepped up to take the blame for the loss — but it wasn’t the kicker who drew those 16 flags.

• On October 24, the Giants played just about their best game of the 2005 season, thrashing their division rival Washington Redskins 36–0. Eight weeks later, on Christmas Eve, with a possible division lead on the line, Joe Gibbs’s team made radical adjustments and shocked Coughlin’s sleeping Giants, 35–20. The main culprits this time were the rushing defense—which had held Washington to 38 yards in their first meeting but this time gave up 156— and the secondary which allowed the Skins, 13th in the league with an average of just 6.9 yards a throw, to gain 224 yards on 19 passes, an average of 11.2 per toss.

• The Giants won the division anyway and played the Carolina Panthers on January 8 in the Wild Card game. The Panthers had the same 11–5 record as New York, but the Giants, in front of a home crowd, weren’t even close to being competitive. This game stands out among Coughlin’s major defeats because it’s the one game in which the team was consistent: The Giants were lousy in every phase of the game. Manning couldn’t pass, netting just 91 yards through the air with three interceptions, and Tiki Barber couldn’t run — he was held to a season-low 41 yards and lost two fumbles. The Giants couldn’t play defense, giving up 223 yards on the ground and 335 yards overall. After the game, Jeremy Shockey made his famous statement: “It was strange. It was like they were in our huddle.” Perhaps the Panthers had simply watched films of the Giants in the second half of the season and seen how predictable they had become.

• On September 24 of this season, in a rematch of last year’s 21–24 loss to the Seahawks, the Giants found themselves down 35–0 at halftime and 42–3 in the third quarter, and went on to lose 30–42. Coughlin found someone other than himself to blame for the disaster: “A team that does nothing but preach and talk about turnovers, we turn it over like nothing maters. It cost us the game.” No, it didn’t. Manning had three interceptions, but so did Seahawks’ quarterback Matt Hasselbeck. What killed the Giants was seven offensive penalties that negated 105 yards worth of offense and a complete inability to come up with the right third down play. The Giants were two of nine while the Seahawks converted an eye-opening 10 of 16. Jeremy Shockey got it right this time: “We got outplayed, and outcoached. Write that down.” And we did.

It’s tough to find the thread that runs through such wildly erratic performances, but in recapping the games, some themes begin to emerge. In big games, the Giants — on offense, defense, special teams, and in the kicking game — always seem to beat themselves. In all of these games, the Giants were sloppy, unfocused, and poorly prepared. Their game was always predictable, and their coaching staff was unable to make adjustments. In retrospect, the fourthquarter debacle against Tennessee last Sunday doesn’t seem like an accident. To paraphrase Branch Rickey, bad luck is the residue of design.

Mr. Barra is the author of “The Last Coach: A Life of Paul ‘Bear’ Bryant.”