When you car is cold, spin the fan around by hand with a certain amount of force and see how far it spins after you let it go. Do it a few times. Then again when it's hot, try spinning it around with the same amount of force a few times to see how far it spins. When it's hot the fan should spin around significantly less than when cold. If that's true, your problem is probably a finicky thermostat or mucked up radiator, so plan on doing a good radiator flush job soon (look for Ayeroll's excellent and thorough procedure) and replace your thermostat and gasket.

If the spinning of the fan doesn't change much with the temperature of the engine compartment, replace the fan clutch as the other posters suggested.

I suppose it's possible that it's your temp comp board starting to act up, but since this problem seems to be tied to real temperature situations, that's probably not the problem. Still, it's always a good idea to jumper the temp comp board so you can trust the readings on your gage.