More on Cold Idle Problem

Forgot to mention that I checked the error codes, nothing but 12's. I also put a 100k resistor across the coolant sensor connector to simulate a cold engine. Did the same thing.

Pontiac 1988 Fiero GT with a 2.8 liter V6. Mileage is 108,000. In cold weather it starts and immediately stalls. If I increase throttle it misses badly and stalls. After several attempts to start, it will start and run, but will not idle. If I take my foot off the gas the tach drops to 400 rpm for a few seconds and then the engine stalls. As it warms up the idle is very erratic, ranging between 1500 rpm and dropping to 400. After the car warms up it runs fine, and will run at full throttle through the gears up to 60 mph no problem. I've run carburator/injector cleaner in the gas, and tried a fresh tank of gas. Any ideas?