Why is it doing this to me?? Overheating problem!!

[89_3SFE]

I've got a 91 Camry V-6 and it has recently been acting up on me. I drove it about 35 miles, and after about 10 the car started to creep past the halfway point. It didn't really move that much until I floored on the gas. I also had to drive pretty slow and pop it in neutral down hills and it eventually went back down to the original temperature. I also had the air conditioning on when this happened, but I know it happens when the A/C is not running, too.

I am not hearing any weird noises or getting any weird leaks at all, and I'm sure the compression on the head gaskets is still good, so I don't really know what it is. Also, the fan kicks in so I'm sure it's not the temperature sensor either. If anyone can help me, please, because I need a good faithful car that'll go more than 15 miles without blowing on me. Thanks everyone!!

[eviltwin67]

Check the overflow resivoir. My gaoge kept doing that to me last year and I noticed that my overflow was empty. I topped it off and it went back to normal.

[89_3SFE]

It's topped off on everything. That's why I'm so lost in this. Everything's topped off, the mixture is good, and I know the fans are running. I think it might just be the thermostat, though.