I had a similiar situation with my 91 300SEL. Unfortunatly the car self destructed in the dealer parking lot. When I noticed the car running warmer than normal, I drove to the dealer to buy a new thermostate & radiator cap. When I parked the car, the gage was about 3/4 high. When I got back to the car, It just made a high pitch whine when I turned the key(no compression). $4500 later, it is running great. The dealer said what started the whole chain of events was a broken radiator neck -Go figure....
P.S. Exhaust bubbles from the coolant resevoir is a bad thing
P.S.S. I agree with the rest of you, this is an extrodinary site!