AN awful blunder by Elvis Grbac, the normally reliable back up quarterback for the 49ers, handed victory on a plate to the Dallas Cowboys, who came to San Francisco badly in need of a win to keep their play off hopes alive.

Grbac has filled in ably for Steve Young, who has missed several games this season from concussion. His services were needed once more when Young retired late in the second quarter, again from concussion after several hard knocks. Certainly good enough to be a starter anywhere in the league, Grbac may well leave next year unless the 49ers shell out lots of money to keep him.

But this was not Grbac's afternoon. He threw two interceptions, including the one that effectively cost the 49ers the game. On both occasions, Grbac was under pressure and tried to force the ball down the middle when he should have known better.

The crucial mistake occurred late in the game, when the 49ers were leading 17-10. The mistake was particularly galling as Troy Aikman of the Cowboys had Just thrown an interception of his own as the Cowboys were poised to score at least a field goal.

But Deion Sanders had stopped in mid route, allowing Marquez Pope to pounce on the ball. The turnover should have snuffed out the Cowboys, perhaps dooming their play off hopes.

That's when Grbac played Father Christmas to Dallas. With Cowboys galloping at him, Grbac flung the ball straight into the waiting hands of Fred Strickland, a Cowboys linebacker. Given a second chance, Aikman made no mistake. A few plays later, Aikman curled the ball over the outstretched arm of a 49er and into the hands of Eric Bjornson for a six yard touchdown that tied the game with 2.45 seconds of regular play left. The Cowboys wrapped up the game with a 29 yard field goal from Chris Boniol in overtime.

This might be the reprieve to buck up the Cowboys for the rest of the season. In another stroke of luck, the joint leaders in their NFC East division both lost. The Philadelphia Eagles went down to the never say die Buffalo Bills 24- 17, while the Washington Redskins were upset by the Arizona Cardinals in a 37-34 overtime shootout.