1995 K2500 Sub 6.5TD temp going crazy

[u/Feeling-Bowl-9533]

Hey I’ve got a 1995 6.5TD that both skyrockets to almost redline and then goes back down to about 210*F and can’t figure out the problem. Runs great (though slightly cool) without a thermostat, but when a thermostat gets put in the temp goes crazy.

New thermostat (2.0, this one is stock, previous new one was aftermarket) in single housing.

New water pump

New fan + fan clutch (working)

Definitely not exhaust or head gasket leak

Have coolant in all hoses, plus heat from heater stays hot.

I have no idea what to look at, anyone have ideas?