99 RAM 1500, Temp gauge went crazy.

Driving today the temp gauge jump all the way to reline. Pulled over and shut it off and popped the hood no leaks and didn't seem to be that hot. Started it back and still showed hot for a few seconds then came back down to normal. Drove another 10-12 miles and then it jumped to redline again. Still no leaks. Got home let her sit for a few hours. Checked the radiator and it was full. Crank it up cold and it temp gauge shows pegged all the time. Any idea's????

If it is not actually overheating I would be looking at the temp sender operation and/or maybe a chaffed/shorted wire from the temp sensor to ground causing the guage to peg out.

The Magnum V8 temp sensor is a thermistor and will falsely read high temperature if the wires short and resistance is low.
If my memory is right 212F degrees is about 640 ohms. The Ram FSM has a table of ohms versus temperature.

The Magnum V8 factory thermostat is a
Sieb/Robertshaw standard grade with steel body and brass underarms.
The brass will crack,
which will cause the thermostat valve to cock sideways to fail in the mostly closed position.

If you have to work on these
take out the 10 extra bolts
and lift the alternator and AC compressor off to the passenger side
so you can work on them properly.
Permatex "Right Stuff" made a good gasket-less seal on the thermpstat housing and teflon tape made leak free seals on the thermistor sensor.

Gauge just showed that it was running hot. Replace temp sending unit. Cured the problem.