Thanks to those who responded earlier - car heats up, then usually cools off when aux fans come on - then heats up again. Everything in new (radiator, thermostat, radiator cap, water pump, switches for aux fans). Temp can hit 120. Main fan clutch has been checked by dealer BUT?? could it be bad and work part time?? How do I check??

Most MB's. i dont know about newer ones, have, for some reason, a really high temperature thermostat, i think it opens at around 105 C or 110 C. So, hitting the 120 mark is very common, usually, it hits that mark if your sitting for a while in hot weather, and then comes down a tiny bit when moving again! You didn't mention the year, so im assuming because its a 500, its early 80's, very common for these years. There are threads in here about the thermostats, and i believe one thread about someone who replaced the thermostat with a german made thermostat that opens at 80 C or 85 C.

Good Luck!