I was pushing my '84 300D up a mountain hard, this was a 5,000 foot climb. About halfway up the mountain, the temperature suddenly climbed and I pulled over just before hitting red line. It took about 2 seconds to go from normal to red line, I saw the needle swinging up.

I checked the oil level, it was fine (I have a bad oil leak so this was my first thought). I couldn't find anything wrong. Coolant level fine, and no boiling. I put on a new water pump two years ago. I gave her a few minutes to cool down a bit, she started up fine, I turned around and coasted back down the mountain basically at idle. Immediately as I started driving down the temperature returned to normal.

Still not finding anything wrong, I went back up, slowly in low gear, and no trouble. This was 500 miles ago, normal driving since then and no problems.

I'm thinking it may have been a stuck thermostat. Any other theories? And assuming it was a stuck thermostat, what could cause that and how can a relapse be prevented?

__________________
1984 300D Turbo 350K 50K WVO (two tank) "Maeby"