Normally when you speed up the motor and the air conditioner "cuts out" it is because it is a little low on gas (R134). THe pump running faster, sucks more, and the low side pressure drops below 25 psi and the low pressure switch shuts off the compressor. You need to put a set of gauges on and test the pressure on the hi and low sides. Do this at idle, then speed the motor up to 1800 RPM and watch the pressure for 60 seconds to see that it maintains proper R134 pressure.