1991 Chevy Corsica 91 Corsica odometer and speedometer work

The odometer and speedometer quit working, for the most part, on my 91 Corsica (3.1 v6, auto trans). Used to be that occasionally they would quit while driving, but kick back on shortly after. Now they rarely ever work. Also when they quit working, the a/c blows hot and the car runs rough and stalls out. I have to keep a little gas on the pedal to keep from stalling when slowing down or stopping. The car only has 48,000 miles, but was sitting for awhile before I bought it. Please help!