Engine Performance problem
1984 BMW 318 4 cyl Two Wheel Drive Manual 149000 miles

The problem I'm having is when i start the engine when cold it starts fine, but when the car is setting there idling it begins, after about 30 seconds, to start running rough and the idle starts going from 1500 rpm to 500 rpm. It sometimes dies and I precede to start the engine again and it still does it. This goes on until the engine starts to warm up then everything is fine. What could be the problem because I've checked just about everything that the manual tells me to do. Thanks

What did you do and what did the manual tell you to do?