[DSG2003Mach1]

with the weird **** youve got going on I would not put it past the fan to be behaving erratically. Unless you're driving at a constant speed of like 50-60+ and it does it in which case the fan should be off at that speed anyway.

Cant remember if Ive said it here or not but my lightning would run just fine then temps would shoot up and come back down shortly after - it was getting on occasional air pocket due to a leaky head gasket. Mine was blown in such a way it was dripping water between head and block so no combustion gasses in the water or water in the oil. It was a very small drip, not even really noticeable looking at the coolant res

if you've got a mechanical water pump I find it hard to believe thats the issue, its rpm driven so it either spins and moves water or it doesnt. I just fail to see how it could stop flowing.

with the thermostat, for testing purposes just take the ***** out for now and be 100% sure. It'll take a while to warm up but once it gets up to temp you'll see.

lastly, consider that the temp sending unit could be bad as well.

edit again - I think you have a super oddball fan issue or a head gasket personally. The temp sender being an outside hope that its something stupid like that