The heater core can't increase the humidity in the air, thus it doesn't seem like the duovalve could be at fault. Whenever this happens, the air from the left side is quite humid. I can smell it, and quite a bit of water condenses on my thermometer.

This effect happens at any acceleration. I rarely put the pedal to the floor. The high-side pressure always plummets though, probably due to the greatly increased air at the condensor. However, I do not have any indication of whether the compressor is running. Perhaps I should hook up a lamp.