88 300E A/C

i'm working on an 88 300E a/c system. the air blows cold but when you drive it for approx. 15 minutes or so, the compressor cuts off and it blows outside air. it doesn't cut out however while idling, no matter how long you leave it idling. the outside air temp is broken and reads -3? (the last number isn't readable. will the outside temp switch cut the compressor out?? or is this likely a tensionor problem?? the system has been evacuated and recharged and has a new control switch. and when the compressor cuts off, you can restart the vehicle or remove the fuse and immediately replace it and the compressor starts again. thanks for the help.