Access-Denied 1 1996 Geo Prizm with good ol 195k miles. Heading out to Wendover, NV (about 120miles west of SLC). Doing about 90mph, AC going, pulling around 4k rpm's. Looks down, and the temp guage was climbing slowly. Turned off the AC, slowed down a bit, and it started to return to normal. I remember Checking the oil before we left and it was about 1/2 qt low. Pulled off at the next rest stop which was about 10-20 miles down the road. Popped the hood to take a look, and everything seems fine. Added some oil. Got back on the road, and continued our trip. Topped off the radiator fuild before heading back to SLC. Everything seem fine on the way back. Granted it was much later in the evening, and cooler outside. All this was about 2-3 days days. Yesterday, I decided to drive the car hard staying in the 4k rpm range, and sure enough, it started to heat back up. So what you guys think. Cheap 10 dollar themostat? Water pump? I plan on replacing the themostat tonight hoping that will fix the problem. Only seems to overheat when running it hard. Replaced the fluid this past fall. Maybe the themostat not opening up all the way when it needs to.