I have gotten so tired of chasing these intermittants that I have used the relay bypass on a number of cars. It can't be easily done on cars with MAS or BM though, so proper diagnostics are still necessary.
If you are interested in diagnosing it look at the A/C article in DIY. There are scope patterns showing what the sensor signal looks like.
Every bad sensor I have found has really been a compressor problem. All were rebuilts and the problem was that the sensor was too far from the crank shaft which it senses.
Bosch Master, ASE Master, L1
33 years MB technician