Does this look like a bad thermostat?
Do they often become sticky if used, removed from car, cleaned, and kept on a shelf?

I changed out a working thermostat after using it for only a few weeks, because I wanted to change to another temperature-rated t'stat. That was fine.

I saved the unwanted thermostat after rinsing it off. A couple months later I installed it into another 240 ('90) that had overheating problems. With the "new-used" t'stat, this car still overheats. Needle goes high after a couple minutes drive - it starts up from the bottom and just keeps going, never stopping in the middle. Run the heater + fan, and it settles at or slightly above the middle, now you can drive all day.

If you remove the reservoir cap when needle is high, coolant spurts out the top of the plastic bottle. So it's not just an electrical wrong reading on the gauge.

I've covered everything else I can think of on this car - I changed out the radiator so I'm pretty certain the radiator is not clogged. I bypassed the temp compensator board and replaced the little voltage regulator on the instrument cluster.

The only other thing I can think of is water pump impeller blades corroded away due to prev. mechanic using tap water?

Considering the cost + time of the parts + work, I think I'll put in a NEW thermostat this weekend - but what do you think?
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Sven: '89 245, IPD sways, electric rad. fan conversion, 28+ mpg - auto tranny. 850 mi/week commute. '89 245 #2 (wifemobile). '90 244 (spare, runs).