idle, gear shifting and accelerating problems

since I took my car for a ac charge at a (auto shop and transmission). it would stall at every red light when I let off the gas. when I took it back they put in a used idle air control valve for free that worked for about a day. symtoms remain the car would not start unles I floor it and keep it reving high would stall so I bring it back and they say they must rebuild the carberator for $240 and this will surely fix my problem and it did not it would then not accelerate properly when I press the gass as if I let off as I let off it will slowly accelerate so I take it back to the shop they replace the spark plugs that helps a little for a day the next day they replace the spark plug wires that helps for another day they I replaced the that worked for a few hours then I changed the distributor cap that bade it run exelent for atleast an hour and good for the whole day then now it is idling very high then low continuously will not stay started very difficult to accelerate having a hard time shifting gears will rev very high before it shifts unles I let off the gas then it will it is a automatic. its odd how all these replacements helped for a very short time. the car did stall while reversing often before all of this and would rarely stall while parked
one last thing it feels like the engine power is cut in half when I switch the ac on and every thime the shop would leave the ac on before telling me to test drive. any ideas most probable first? sure appreciate it.
I believe the van is a 8 cyl but surely not a v an its a 1994 g20 sportvan