Petaling, what you describe with your A/C sounds like an excessive pressure situation which will trip the high pressure cutout switch. The compressor will stay off line until the pressure drops and the switch closes. The fact that at speed and at a lower ambient temp, it works indicates that it is a combo of heat and pressure working against you. Again, it will stay off until the system cools enough to let the high side pressure come down. You've got either too much freon or one of the system sensors is malfunctioning. Make sure they check the high side pressure at idle and under increased heat. I'll bet it will be above spec.
1987 BMW 325
[This message has been edited by jeffsr (edited 07-12-2000).]