Sitting in traffic, my car's thermostat goes up to the red. If I put it in neutral and rev it, it goes up a tiny bit, then suddenly drops back to normal. Is that the thermostat sticking, or something worse?
Vehicle is a 1991 Mercedes 300E
It very well could be the thermostat. However, did you check the obvious things, like coolant levels and make sure you don't have any air pockets? I had an overheating issue that would manifest after 20 to 40 minutes of running the car, which was due to an air pocket.
Usually if it overheats at idle you check the radiator fans, radiator fins bent or clogged, but I have heard of a bad secondary fan causing this issue when high rpms would make the temp go down. Make sure the fan switches are working properly. I have heard that if you put on the A/C it forces both fans to run, regardless of the temp fan switch on some cars, so that might be a way to bypass the fan switch if the fans don't run.
Here is a thread on Odd Overheating Problem that lists a lot of possible things with a similar problem on other types of cars.
Another thing that I experienced recently. I was seeing higher than normal temperatures on my car and the symptoms were similar to a sticking thermostat, which I had just replaced. What the problem was low coolant levels due to a faulty radiator cap, of all things. I noticed misty steam coming up around the hood at idle after the car had warmed up. Then I saw small amounts of water and coolant at the top of the radiator, but I couldn't find any cracks or leaks anywhere. The replaced radiator cap fixed the issue. My point here is observe and note anything out of the ordinary as a clue to the problem.
Need more details. Have you done any related work recently? When did the problem start? How many miles are on the vehicle? Are there any other clues you can give us or is this all there is?
The tell-tale sign is that the coolant temps rise into the red when the car is at idle (low RPM, little air flow across the radiator), but when the car moves the temps drop back down to normal.
So what could cause this? A couple of possibilities:
low air flow
Are the radiator fans turning on when they should? According to eBay, your car has an electric fan and a viscous-clutch-based fan, both should be engaged and running if temperatures are climbing.
low coolant flow
Could a stuck-closed thermostat be the cause for inadequate flow? Yes, but there are other more likely causes:
a failing water pump
flow restrictions (e.g. clogged radiator)
Also, I'd expect that a stuck-closed thermostat would mean that the car would still be warmer than normal when moving.
My buddy had such problem on his MB 190E.
On engines like 190's (and many other Mercedes cars of these years) there wasn't an electric radiator fans, they used something called Viscous coupling unit that transfers torque to the fan depending on the difference in angular velocity between the engine belt and the fan itself. So, primary, standing in the traffic jam may require you to put some revs temporarily to slightly cool down the engine. BTW, my buddy have solved his issue by replacing the old radiator with a brand new one and changing the coolant. You may also need your visco-unit to be checked/replaced.