Originally Posted by LS1'edSSE
Actualy its supposed to do this. What happens is that the A/C system "decreases" output to free up more power, when the car is under load.
Under WOT it may cut out the compressor temporarily, but the vacuum reservoir/booster is there to ensure that the settings don't start changing by themselves just because the vacuum went away for a few seconds (or more).
I'd start by finding your vacuum reservoir under the hood (for the '93 and similar, it'* the large squarish plastic box on the firewall just inboard of the master cylinder vacuum booster) and check the vacuum line connections there. Look for any disconnected or broken lines. You should also not hear any continuous wheezing or hissing noises from the dash, except perhaps when changing the vent settings.
Basically you're either losing your vacuum via a leak, or you don't have enough in the first place due to excess backpressure and a general lack of intake manifold vacuum.