I think that I would try a new thermostat first.
I was having a problem with my truck running a little warm a while back, and decided to try a Motorcraft thermostat, rather than the style commonly found at the "box" auto parts stores. So far it has worked flawlessly. They cost about double what a typical thermostat usually costs, but you can see right away that they are better constructed.
Rockauto has them for about $15.