The engine shut down while on stop and go traffic. Tried to restart but the throttle was weak and shu...

If the car is having trouble maintaining idle (the tach dips and car dies) the common causes are the idle control valve and camshaft position sensors. The cam sensors failing will be indicated by a check engine light.

The ICV can removed and cleaned with good results (I think there is a write up on bimmerfest) or purchased new for about $100.

While you are in there check the intake boot that is Y shaped for cracks, it's prone to fail and will mess up your idle too.

read up on the major forums and you'll get more info