Fuel mixture problem when cold

1992 Sable 3.0 engine with 155,000 miles. Starts when cold but dies as soon as it's put in gear and accelerator is depressed. After it has warmed up for several minutes, the car runs fine.
I have replaced the temperature sensor and cleaned the air flow regulator that is supposed to adjust the fuel when the engine is cold. No improvement.
The thermostat may also need to be replaced since it appears to be running warm (about 2/3 of the way up the gauge when warmed up).
Any ideas?