My MB 200 overheats suddenly from time to time. For no obvious reason, the temp guage starts rising at a fairly rapid pace up to 110 C or even higher from its "normal" level of 80 C. Sometimes, but not always, it drops down partway again. I don't understand why the temp should change so quickly. The radiator has been flushed out and the thermostat replaced. The mechanic found no leaks in the cooling system, yet the water level in the radiator seems to drop alarmingly fast. Any thoughts?