Temp Gauge fluctuates big time

[tdangle]

Installed a new sending unit as the old one completely died. Now the crazy thing goes from normal warmup to peg and back in a matter of 15 seconds while driving. The engine doesn't seem to be running hot, I've checked it with a temp gun. It can idle for 1/2 hour and be steady, drive it a few blocks and it's all over the place. I've replaced the thermostat no change, I have a coolant reservoir and it keeps the rad full to the top. Before the sending unit died (no reading at all) it was solid reading all the time even with the AC on in 100+ days