1989 Nissan 240SX over heatting

I have a 1989 nissan 240sx 2.4L and anyways I been finding in the last week that everytime I went up a big hill with my car my car would start to over heat and then if I went any where else like down hill or just on flayt surface it would be fine then found about 3 days ago it will do it everywhere so ive been told to change my thermostate and rad cap so I did that last night and today went for a road test and didnt over heat for lke 15min and then I left it for a couple hours and same thing still over heating checked my oil and it looks great and my exhaust isnt blowing any smoke so I dont think it coul be head gaskets but any thing any buddy can help me with