Hey guys and gals. So I have read through most of the other threads on overheating issues and have read the Everything you Ever Needed to Know About Cooling Systems thread. And I'm still a little unsure to what my problem is.
So here is the problem, my car started overheating a couple of weeks ago on a longer highway drive (about 2 hours). The car's temperature was fine but then after about an 1 & 1/2 hours of driving on the interstate my car started to over heat. The temp gauge would slowly start to raise and then it would go down a little. Then back up again and then back down. When I noticed this I got worried and cranked the heater up to try to dissipate the heat. This didn't work. Then I downshifted to 4th gear and got the revs past 3500rpm and the temp gauge went right back to it's normal level. Then when I put it back in 5th gear it started to overheat again. So after this happened I took the nearest exit and stopped for a little while. I notice the water in my overflow tank was much higher than normal and there were bubbles coming out of it. I sat there for probably 5 minutes and then started the car and drove the rest of the way to my destination without another problem.
Then on the way back from this destination, which would be another 2 hour drive, nothing happened. The car never once overheated. So I didn't worry about it too much since then.
So I had driven the car for 3 weeks and never had this issue again, but all this driving was for an hour duration or less.
Well last weekend I went on another trip that took 2 & 1/2 hours to get there and my car started to overheat again. Just like before, after an 1 & 1/2 hours the car started to overheat. Again if I downshifted the temp gauge would go right back to normal. I pulled over, looked in the overflow tank and again it was higher that normal and had bubbles coming out of it. I waited 5 minutes and continued on my journey without any more problems. Until I was driving back from this destination and this time the car didn't start to overheat until about 2 hours of driving. Again with the same symptoms. As long as I kept the revs up the temperature was fine.
The car is a 2002 2.5RS with 108,000 miles on it. Performance modifications include Cobb spicy cams, full exhaust, intake, pulley, and a lightweight flywheel. There is no leaking, at least that I can see. The headgaskets were replaced about 15,000 miles ago by a reputable dealer with a mechanic that I fully trust. The timing belt was replaced about 8,000 miles ago, along with a coolant flush. Again by the same mechanic.
So can someone please help me figure this out?
Sorry for such a long post. Hope it's clear enough to understand.