Gentlemen. I think that you have a bad thermostat. Your temperature sensor is probably still in the expansion tank and not in the engine where it should have been relocated to.
If it was in the block you would have seen temperatures go up then down then up then down then finally on the ceiling.
Sounds like a classic thermostat failure to me. I certainly hope it is too. It's a hell of a lot cheaper to fix then a head gasket or a cracked block.
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