Temp Gauge Problems

[RicciDSM]

Ok so the car is a 97 Talon TSi FWD Automatic.

Just recently replaced my radiator and alternator...Everything has been good but now while driving after I hit 40+mph my temp gauge will start to rise but then drop down to normal if I come to a stop or drop it in neutral. I tried turning on my a/c while the gauge was up and the second the fans kicked on it goes back to normal...

Theres plenty of coolant in the car so I have no idea what this could be. Any ideas? Not sure if its my gauge acting up or a real problem...

 

[nek]

If the gauge is fluctuating I doubt thats the problem. The system can overheat from tons of things including, but not limited to, water pump, tstat, rad cap, headgasket, poor/obstructed flow

 

[RicciDSM]

If the gauge is fluctuating I doubt thats the problem. The system can overheat from tons of things including, but not limited to, water pump, tstat, rad cap, headgasket, poor/obstructed flow

Not sure if this could be part of the problem but my valve cover gasket is going and has been leaking oil for quite a while, I just havent had a chance to fix it. I wonder if that could be the problem? Due to oil dripping out onto my heat shield and all?

I checked all the basics though. All electrical connections to fans etc are fine, no blocks in the hoses going to rad, rad cap is on, plenty of coolant in there. Im not sure what else it could be. How would I know if my thermostat is bad?

 

[nek]

Truthfully I know the term is used way to much on here for overheating. But when you only overheat at higher mph, its usually because of boosting. When overheating due to boosting, its usually the head gasket leaking.
Even if it is the headgasket, you shouldnt just jump into it. I would fix the cheap things first that are getting old. The rad cap is cheap ( go oem not oe from pepboys), and get a 180 tstat with a gasket. From there, if that doesnt fix your problem, now you narrowed it down and it only cost you 15 bucks.
Do a compression test and a leak down test. I think my hg is bad but my compression test came back with flying colors. I need to get a leak down test to be 100% sure.
If you have a compression tester, I would do that asap.