It happens ever so often, and when it happens in playoff games the focus becomes ever sharper. For reasons nobody seems able fully to explain, teams - like the Colts today - have wonderful instincts for attack while gaining a lead and then creep into a shell while defending it.

And more often than not, that strategy fails, as it did today in Baltimore's 37-31 overtime lose to the Oakland Raiders in the American Conference playoffs. It was the only flaw - but a large and fatal one - in an otherwise splendid performance by both teams.

"We don't have a lot of success when we sit on it," said Lydell Mitchell, 'and I guess you could say that's what we did. One running play didn't work, and then another running play didn't work. Maybe when you get a team down, you've got to keep 'em down."

Exactly. And when the Colts got the Raiders' down by three points with nearly eight minutes left in regulation they did the one thing their fans and some players considered dreadful: trusted the defense against Ken Stabler.

Down 21-17 with 7 1/2 minutes left in the third quarter, the Colts struck with the flair that best features quarterback Bert Jones. On first down on three straight possessions Jones passed, for 19 yards, 20 yards and 16 yards - and the Colts took the lead.

With that lead, the offense all of a sudden stopped being daring, passing only in predictable situations and mustering just one fist down in the game's final 20 minutes. What that did was give too many passes to Stabler.

"Field position was a little better (when the Colts ran)," coach ted Marchibroda insisted. "And you've got to hold onto the football. But I think their big plays, their capacity for the big play, was a bigger difference than what we did.

"You can't move the ball for touchdowns every time, you know."

And until the final two minutes of regulation, until a second-and-10 from the Raider 44 to be exact, it seemed as though that close-to-the-shoulder-pad philosophy would work.

Then Stabler once again looked for tight end Dave Casper in a critical situation. In fact, Casper was not where he was supposed to be at the time. Stabler sent the ball on a dangerously high are more than 30 yards down the middle anyway.

Predictably, Casper broke free from linebacker Tom MacLeod, caught up with the pass, changed direction, caught it over his shoulder and stumbled several yards more to the Colt 14.

The 42-yard play allowed Errol Mann to send the game into overtime with a 22-yard field goal. Stabler had completed just one of his last eight passes previous to that completion.

On the first possession of overtime, Colts end Raymond Chester had no Raider within seven yards of him far downfield on third and eight. Jones did not seem unusually hurried, yet overthrew him badly. The Colts did not offer a serious offensive challenge thereafter, in part because Jones suffered an injury to his left shoulder.

As Marchibroda suggested later, the Raiders have been through this sort of thing before and the Colts have not. That might have been just the necessary difference, or it seemed so on the two overtime plays that assured victory for the Raiders.

Oakland needed 19 yards on third down from the Colts 45. Cliff Branch got 19 and the length of the football with a diving over-the-middle catch. The play for the winning touchdown perhaps best illustrated both teams under extreme pressure.

The Raiders had second and seven from the Colts 10. All they needed was to run two more plays and set up Mann for what would amount to a long extra point. Instead, they took a chance. Not a large chance, but something with a bit of risk.

It was a roll left, Stabler having the option of floating the ball to Casper in the left corner of the end zone, if he was open, or floating the ball over his head and out of bounds, if he was not. Of course, Casper was open - for he always seems to be open when matters get serious - and Stabler hit him perfectly.

Jones said he suffered a pinched nerve in his left should on the next-to-last Colt possession "that send bad vibes down my arm. But it turned out all right."

"Turning point? There really wasn't one until that last pass, 'cause the game seesawed back and forth so much."

Of the decision to turn conservative, Jones said: "They were taking away the pass. You've got to play ball the best way you can."

Nearby, Marchibroda was softly tossing a roll of tape and saying of the Colts' loss in the first round of the playoffs for the third straight: "Sometimes the thing you want most is the hardest to obtain. But one day we'll reach that goal - and then we'll appreciate it all the more."