Overheating Problem

Ok, about two weeks my thermostat gauge was going up whenever I reached higher speeds, like over 45/50. But, I could start the car and let it sit, and the needle stayed at the normal level. I took it in, they said it could be the head gasket, but were kind of puzzled by it not overheating when running idle. So anyway, I got the radiator replaced, along with a new thermostat, and a coolant flush. The engine still overheats. But the needle flucuates. I was going up a hill with the needle up real high. I switched into 3rd gear (automatic) to get up the hill and the needle went down to normal level. I checked the radiator and coolant after driving about 10-15 minutes at about 50 mph and noticed the coolant steaming from the reservior. Not a lot, but semi noticable. The engine doesn't smoke despite the needle reaching up to beyond the H. I can hear boiling when I turn the engine off too.

Any help is appreciated.