Saab 1999 9-3 Stalling

(this is background info: the real problem is stated three paragraphs below!)

OK, we LOVE our Saab. But admittedly we also HATE our Saab! We bought it three years ago when it was two years old from a dealership in Maine. The first day we bought it broke down and had to be towed. And then again three times in the next two weeks! And for the last 3 years, we've had so many episodes with this car!

Finally about 2 months the whole engine finally blew. (Thank god -- it was on the verge of going for awhile). And we replaced it with a used engine, because we couldn't afford a brand new one at the time. After the used engine was put in, it ran SO GOOD. It was like a brand new car. We fell in love with it all over again! Except that now we are having more problems! There was an oil leak and the head gasket had to be replaced. Luckily the new engine is under warranty for six months so we didn't have to come up with the $1000 to replace the head gasket.

Now, the most recent problem, which is where I need some help:

Ever since the engine was put in, whenever the car is at a stop and it has to idle for more than 10 seconds, it starts to stall. Sometimes it will stall entirely, sometimes it stops itself and recovers. We took it back to the people who put the engine in, THREE times and they couldn't find the problem. Then we took it to the dealership and they couldn't find the problem. So.... our ideas are: either the IAC Valve. Or the o2 sensors. Or the computer itself. The check engine light has come on twice and each time it gives an error code of "engine overrevved." But we are so careful with that car, and never tack it above 2. So it couldn't be overrevved. We did replace the o2 sensors a couple of years ago and the problems were actually quite similar as to the problems we are having now. Though it's all a bit vague to me. I am not sure if this new used enegine we bought came with it's own o2 sensors or if they kept our old ones or not.

And this week a new problem crept up that might be related. Right after we FILL the tank, it runs REALLY rough for the first 30 minutes of driving. It almost feels like the car isn't getting fuel, or it gets cut off and the car jerks a bit when driving.

If anyone has any ideas at all, please let us know. The mechanics can't seem to give us any information, and before we spend lots more money I wanted to check with the Saab community and see if anyone could kindly offer me an educated guess!