90 300SE stalls at idle after 5 min, then ok after 10
 

I have a problem that I hope you guys can recognize easily.
The car starts fine and idles fine when cold about 1100RPM. After about 5 minutes of running it idles poorly and sometime stalls when i stop. However after it's completely warmed up, it idles fine around 700. It runs smooth at all times at high speed.
When it's about to stall, I notice the economy gauge jumps up about 20%(less economy), When it's about to die, the RPM's drop to about 500, if it doesn't stall it jumps up to about 1200-1400 momentarily. Again, it runs fine when completely hot.

Sounds like a problem with the air flow position sensor, try searching here for similar stalling issues.

Thanks, I'll look into that, but if the AFS had a problem, wouldn't it cause problems after the car is warmed up?

The air flow sensor- Is it the black rectangular piece that is about 3"x1.5"x .5"? with a connector on it. i couldn't see how to remove it? Is it a pot that I can test with an ohmmeter? Have you seen this behave strangely, but then work well when the engine is hot? I guess i"m not convinced that it's the problem.