Kinda hard to diagnose but I'd start with the clutch fan. The next time the gauge reads over 110*C, pop the hood and stop the engine using the stop lever on the IP. The clutch fan should stop within a blade or two. If several blades pass or it freewheels, you've found a culprit if not the culprit.

Do a search on the 3-pin thermo switch on the upper radiator hose fitting. It looks like a thermostat housing but that ain't where the thermostat is in your engine. Anyway, I think that's the switch that tells the auxiliary fan to turn on a 115*C or some such setpoint. Do a search to confirm which thermoswitch controls the fan. You can ground one of those pins to test the fan.

If this occured with the AC running then do a search on which of the receiver/dryer pressure switches triggers the fan.

Sixto
93 300SD