i have a on/off switch that controls the VAC. The switch is off and the car starts but drops to like 400 rpm and the engine shakes and its not good. When i turn the switch on the computer fixes it and it revs up to about 900 rpm. This is all when the car is completely warmed up. It stays around 900 rpm, but starts reving up and down about 200 rpms each second. I checked vacum hoses for hissing and leaks and the gastket on the intake manifold. I jsut cleaned the throtles and it really helped alot. But does this point to a bad BAC valve?

 

What you discribe is what the call idle speed hunting. There was a recall in 1985 for vehicles with a prior vin # jm1fb332 F0889474. This recall installed a delay valve between the dynamic chamber and vent selenoid valve. If you have this recall like I did and still had the same problem, which I spent many hours trying to fix it.
I took carb cleaner and cleaned out this delay valve.
I also replaced the check valve behind the air control valve. This one was the problem I think since I did both at the same time. This check valve was all full of carbon and hard as a rock with only 60K miles on the engine.

The replacement was still available at the dealership.

Good luck!