I have a 2002 Cadillac with 70000 miles on it. I can let it idle for an hour and the heat gauge stays where it should. When I drive it, it stays there about 20 to 30 minutes and then jumps quickly to the max temp. The radiator fluid holder was low so I filled it up and it is staying full. It seems like if I hit the high speeds, it goes down. Does this sound like head gasket?