Air Conditioning Problem

I have a 1997 P38 RR and it has some A/C problems.  At first it was just not changing temps on one side.  It was always stuck on hot.  So i just got some blend door motors in to fix that problem.  The new problem is that on the side with working blend motors, sometimes when just cruising, the AC will go from cold air to hot air.  To get it back to cold air I have to step on the gas to make the vehicle almost redline.  When I do that, I hear a hiss from front of the car, and the air goes back to cold air again.  It gets annoying to have to keep redlining the vehicle to get the one side to stay where it's supposed to be.  Anyone ever had this same thing happen?  I just want to know am I dealing with blower motors, compressor?  what?
 
Thanks for the help