I am not trying to add more rain to your parade, but I've had a very similar problem with my off-topic 96 Lincoln Town Car (i didn't have this problem with my benzes...yet...). It began overheating a little bit at first, I was going nuts trying to figure out what was going on. I didn't overhead badly and only did it occasionally and was running like that for months until about 3 weeks ago. It was about 95 degrees outside and I was in traffic when temperature shot up to the max. I let it cool down, then restarted again and drove it back home - temps were OK. Then a day later - temp back to the max. I said enough, went driving to the shop - when I arrived, temps were close to the max, I suspected anything but headgasket (because I was checking both the coolant and the oil - no signs of any leakage were present), but on my final drive to the shop I noticed roughness on take off. In any case, after checking compression - it was the head gasket. So draw your own conclusions, just a story here to add to your deliberations, better safe than sorry (warped heads and all).