i have a 2003 G35 Sedan. 138k Miles

so i was driving and noticed my temperature gauge going up. I was on the freeway going about 70mph with the AC unit on. when i saw it about 3/4 way up i pulled over and waited, i popped the hood and noticed my resevoir cap was broken. I replaced it assuming it over heated because there was no pressure. Next day i was driving AC on, going about 45/65 and it did the same thing. I let my radiator cool down then opened the cap and made sure it was filled up correctly, i let it run without the cap until it about overflowed then screwed it back on. I have felt my hoses they have pressure, i have nothing leaking underneath the hood. Could this be a thermostat? I am replacing that today. But i have never seen the car go ABOVE 3/4 . Now when i drive i keep the heat on and it never seems to really do that. Any suggestions? besides drive with the heat on?

 

Test fans for proper operation, same with thermostat, flush cooling system, clean debris from radiator, pressure test radiator cap.

Fix anything that doesn't pass, if it still overheats replace the radiator, unfortunately that's one item that you can't really "test".