Help!!! Temp Guage???

I have a '98 Ford Explorer. I just recently noticed that my temperature gauge goes up and down. This occurs mostly when i go up hills.. and then it returns to normal after i either level out or go back down hill. Also, i seem to loose acceleration and power as the temp guage begins to climb. The water in the radiator and resovour tank is fine, and holding steady. Could it be the fan clucth or....??? I need help! Tks!

youve got nothing to worry about, i was too scared about the temperature in my 94 explorer rising and dropping it was wierd, but its only your thermostat. 5bucks for the thermastat and 3 bolts to change it. its right behind the radiator.