You need a set of gauges to see your low and high pressures. Chances are, your high-side pressure is running way up there and the high-pressure switch is kicking the clutch on the compressor off. This can be caused by lack of air-movement through the condenser in front of the radiator - check to see if your electric fan is kicking on when you engage the A/C. I found this on my '93 190E 2.3 - I forgot I had removed the fuse for the electric fan when I was having alternator issues and the compressor would kick off when stopped at idle. I put gauges on it and found my high-side pressure was running well over 300 psi and would kick the clutch for the compressor out, and re-installing the fuse dropped it down to around 225ps.