2003 Ford Explorer thermostat

While Im driving the temp. Gauge goes up to the H for about 10-15 sec. And then returns to normal. This happens about every 3-5 min. I have checked the radiator fluids and they always seem a little low, so I fill them up, turn the truck on and everything runs fine. No leaks or anything until I start driving. The temp gauge goes up and I start smelling radiator fluid, so I pop the hood and the reservoir is low and there is fluid leaking under the right fender, however there is no sign of leakage anywhere on or around the reservoir or the radiator! My intuition tells me its the thermostat, however with the advancement of technology it could be the tire pressure that's causing it for all I know. Anyway, any advice? -Rory-