I just replaced the thermostat on my '90 240DL, flushed the radiator, and did my best to clean out the gills by hitting the radiator from the back with high pressure water. But, my car is still overheating.

The symptoms are a little odd, which is why I'm posting again.

Today I drove city streets for 30 minutes twice- no problem. I went out for the evening (city streets, 20 min.), no problem. I go home for the evening- big problems. When I come to a stop, one of two things happens. Either the temperature gauge drops to 6:30, or jets up to the 10 o'clock position. By far most of the time it goes high. I start driving again and the car cools down.

Sounds like a fan clutch problem, right? To test this theory, I take it on the freeway. Highway speeds lead to immediate overheating, almost to the red. I get off the freeway, turn on the heat, and watch the gauge sink back to 9 o'clock. To get home, I get back on the freeway- and guess what, everything is almost fine until I get off, when it starts overheating again.

I didn't see a leak in the water pump, though I don't know exactly what to look for. This was the first time I've ever flushed a radiator, I may not have done it perfectly. If I have a leak, it's very slow.

To make things more confusing, the overheating was even worse (happened faster) before I flushed the system.

I've noticed that the overheating is mostly associated with accelerating (ok, the car is working harder), and sitting still. What is going on with my car? Did I blow some electrical component? Should I flush the radiator again?

You guys have been good to me so far.
Thanks for the help, and have a good 4'th of July,
Chris