When the air-conditioning compressor is engaged it drops the engine RPM by around 200 - 300 RPM. This means the engine would normally stall but engine management system increases the RPM to compensate, pretty much along the same lines as how the automatic choke works. I can only guess that it is compensating too much - do you know if the engine is fuel injected or carburated? If it's the latter it would make more sense why you're having problems.