1990 Audi Quattro Car stalls, low mileage

Please help me find out why my car stalls..

When weather is really cold, and the engine stays cold, with highway driving, it works better. When the weather is warm, or the engine warms up, the car stalls every time I come to a stop, unless I keep my left foot on the brake, and keep the engine revving with my right foot, at 800+ rpm. With warm weather, highway mileage now has dropped from 20 mpg to 10 to 13 mpg, and the plugs are becoming fouled.

My car: 1990 Audi V8 Quattro, 3.6 L
VIN# WAUKE5440LN024906