I have an interesting problem with my 04 aveo. The coolant temp gage normally stays just under the half mark. Now it climbs when the engine is under heavy load such as climbing a steep hill or running at or above 2500 rpms. At idle or driving slowly it stays where it should. The car has 202000 miles. I have previously replaced the oil pump (went out a few years ago), water pump (with the oil pump because I was there), oil sending unit, water pump again along with having the head decked (no valve work just resurfaced),and thermostat. The car ran flawlessly after I had the head resurfaced. Had a coolant hose burst. replaced that and the radiator, now I'm having this issue. I'm not sure what could be causing this. I'm thinking maybe the thermostat or the head.
Anyone have a similar problem? Any ideas or thoughts greatly appreciated.