someone may have removed the vac cannister (a plastic ball) or the one way checkvalve on the HVAC line from your SC/UIM vac port.
look for the line that runs into the harness/firewall passthrough and toss one in so that the engine can only suck and not blow (also good to check it doesnt leak (suck on it) in case someone broke the vac cannister (not all cars have vac can fyi)
whats happening is that yer manifold vac drops while accelerating and since the checkvalve isnt there the HVAC system looses vac and the actuators move position. this happens to the digital/vac systems as welll as the older mechanical vac systems.
