Could this be the AC Clutch? As the compressor heats up, it could be doing something to the clutch.

Quite frankly, if no one here knows, you might need to head to a shop....


98 Silver Frost SVT 97 BMW 540I Sport, six speed "Blue is for sky, black is for soil, and white is for simplicity, purity and hope for the future" "A coveted car should never stunt your life, but should make it more rich and interesting."