Possibility of a bad sensor. They work off of resistance, the more you heat it up, the less resistance it has, and the gauge rises.
Another thought, if you're not having any overheating issues. Do you have all the factory heat shields still on the car? If not it will heat the engine bay up in a hurry, even though your motor may not be running all that warm. Should be a shield on both manifolds and the crossover pipe if its a v6. If its a 4 than I'm not sure how many shields it should have or where they are all located. One way to check your sensor is to hook a meter up to it, switch it to Ohms and ground the negative meter lead, and placed the positive lead to the wire off the sender that goes to the temp gauge. Start your car, you should begin to see the resistance drop the longer the car is running. There is a post somewhere on here that tells the heat to resistance ratios to compare with what your gauge is showing.
BTW, you live 45 minutes from me