My 1988 240GL n/a did the same thing - on a 200-mile trip, halfway along the temp gauge dropped to about 7:30 instead of the normal 9:00.

First thought was that something blew and the coolant level was down below the level of the temperature sender. Pulled into a farmyard and got a water hose ready. Full boat, no missing coolant.

Drove home, checked the thermostat. Voila! In the rim of the thermostat is a small hole with a little pin in it. They put it there to allow air bubbles to get out, and instruct to mount the T-stat with the pin at 12 o'clock. In my case, and maybe yours, the pin had dropped down and was jammed into the mechanism and kept the thermostat from closing.

I re-installed the T-stat, used Perma-tex on the old gasket, and set the pin at 10 o'clock. Problem repaired, Zero cost, 2 hours work.

Good Luck,

Bob

:>)