Make sure coolant level is ok first. It sounds like a sticking t-stat. Those are easy to replace. If that does not fix it, I think it would be a gage or sending unit problem. Try tapping on the gage (slim, I know) You might also try putting the heater on full blast, that should cool it down some. I really doubt that it's a water pump. It wouls likely o/h all the time plus it usually makes noise or leaks visibly. Oh, you might double check that you don't have coolant in your oil (milky) just to be safe. Hope this gives you a start.
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