My temperature gauge fluctuates. Today it went to H. It wasn't over heating or smoking but it lost power. I pulled over, let it cool down then was able to drive home. The gauge was at about 2/3rds. When I got home I turned the car off then immediately turned it back on and the gauge read cold.?

Sounds like a defective thermostat. Car is probably over heating and gauge is accurate.

How is the coolant level?

What kind of car is it?

Sounds to me like it could be a head gasket problem.