A/C compressor stops after running for a while

[creine]

I have a 1984 190d. When i start driving in the morning, everthing seems to work perfectly, the compressor kicks in and out as the coil cools and reaches the proper temp. The problem comes mostly when i have been running for a while. as long as i am going 1500 rpm or better the compressor works perfectly. as soon as i stop at a light it will cut out. The only way to get it to go back on is to completely cut the car off then re-start it. I thought it might be the rpm sensor on the compressor not reading correctly, but it does work at higher rpm's. I have checked all wires on the compressor, i have put a new belt on, and also changed the shock on the tensioner wheel to make sure that the compressor is not slipping. Does anyone have any ideas? what i am doing now is when i am getting ready to stop or slow down i turn off the system until i start going again, then i put it back on. the only problem is that driving through town in traffic, the system has to stay off more than it runs. so why run it at all, I live in south louisiana, so right now its like 90+ degrees and climbing......HELP, before i die of heat .....