Car is a 1982 turbo, engine is basically OEM stock.

On these cold (well, very cool anyway) mornings, my routine is to go out and start the car, scrape the frost/ice off the windows (it doesn't live in a garage), go back in the house to get the stuff I forgot, and then go back out to drive away.

The issue is this: When I first hit the starter, it fires right up, no prob at all. However, after it's been running (at idle) for about 3 - 4 minutes, the idle goes into an oscillation - varies between about 500 and 1500 rpm (back and forth, back and forth, etc). When I get back to the car (usually it's still in this oscillation mode), I can drive away, and it acts perfectly normal. Once it's warm (even just a little), the idle goes back to a normal, a smooth 900 rpm.

So... I assume there is something in the fuel circuit, between "very cold/just starting" and "nice and warm/ready to take on a BMW" that isn't functioning properly.

Anybody familiar with this?