The A/C has a sensor that measures pressure in the system and only then will it tell the clutch to engage and the compressor to activate, it seems that your unit is on the edge of that pressure reading for any number of reasons, sensor going bad, minor leak, when the engine compartment heats up it changed the ambient atmosphere and initiates a pressure changed under the hood, if it was my car I would first locate the sending unt along the A/C system and clean all contacts and run the A/C, if not you have to take it in and have it charged and checked for leaks, unless you have experience and the equipment in A/C do not tackle this job yourself, let us know what the outcome is.

__________________
Jerry

2004 SL500 Black 55K Miles
2002 s500 Black 90K Miles
1986 300E Gold 233K Miles
1966 Twin Cessna 320D
1989 Jeep Cherokee 233K Miles
1994 Ford Van E150 399K Miles