The temp gauge on my 2010 Aveo LS keeps jumping up to about 3/4 and then within 5 seconds goes back to normal temp. It also starts running cold and then returns to normal in the same fashion. This happens only after the engine has been warmed up fully. All fluid levels are full. Also the car has trouble starting when cold. It takes 2-3 tries to get the engine to kick over and once it does start it initially stumbles. Took the car to a mechanic and they think it might be the head gasket but want to run some tests first to confirm. Anyone have any idea what might be the cause of the problems? Car has 40000 miles on it.