crazy temp gauge

[tdangle]

The sending unit died so the temp gauge read stone cold all the time. Verified it was the sending unit by grounding the wire and meter pegged. Replaced the sending unit with an AC Delco replacement. Was reading crazy, would peg and then drop to 1/2 way within seconds. Replaced the sending unit again, figured I got a bad one. First I verified the resistance vs temp of water and everything seemed OK. Installed the new one same thing. Replaced the thermostat thinking it was a problem. No change. Drilled 1/8" holes in thermostat thinking I had an air bubble in the system. Seemed to work OK, but I drove it today and gauge would read 1/3 way up and within minutes or quicker would peg the gauge, than as I watched it would go back to normal. Checking the engine with a thermal gun, it is running 185-195 or so which is normal. 180 thermostat, coolant reserve bottle, good rad cap 50/50 mixture of water and coolant.