[tdangle]

Last year, I had the same issue. The sending unit went bad, I replaced it and started acting the same way. Cold when you start and then would climb to hot peg and then drop to "normal" Temp gun verified engine was not overheating.
Tried 3 different sending units, verified the sending units with water heating on the stove and charting the resistance vs heat of the water. All of them were OK, verified the operating range of the gauge with a resistor box to ground. Totally stumped. Even went so far as to remove the head thinking I had a blockage in a water passage. Also disassembled the water pump looking for the cause. Then started worked all by itself Still don't know. Now have a new motor with vortec heads, got a turned down sending unit and all works perfect. Only thing different with my new motor is vortec heads require a coolant bypass hose setup.