Overheating question

[garfield3887]

I bought a used 2002 neon about a month ago, around 80k miles, was doing great. One day i was going down the highway around 75-80 with the A/C running and it started overheating! I shut the air off, pulled over and let it cool, drove it home with no AC on, and it ran just fine. I flushed the cooling system (old fluid looked like they had just kept adding to it and never changed it for scheduled maintenance ) and filled it back up! Took all the precautions of burping it and getting the air out. Haven't lost a drop since then, which was about a week ago. It does fine with the A/C when i drive to work (about 15 miles, 65-70 the whole way) both in the morning and the blazing sun in the evening on the way home. But when i take it out on the interstate and am running the A/C g going 75+ it likes to over heat! Any ideas? Thermostat? Fluid temp sensor? Air pocket?

Before the first time it overheated it ran just below or right at the lower 1/3 mark on the water temp gauge, ever since it's ran right at the 1/3 or just a hair above it... Just a little frustrating...

[OB]

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