Cooler thermostats do not cure overheating problems.
Look deeper into the problem. First of all, jump around the fuse box and see if you can get the fans to run directly off the battery (You DID check the fuses, right?)
Next, see if the radiator is blocked in some way by debris. Blow out the fins with a hose.
If the car is not using coolant, the problem usually results from air flow around the radiator. If it is internal, removing the radiator and having it boiled out might be in order.
If it IS using coolant, a blown head gasket is probably the problem, not counting, of course, the upper manifold failure which seems to be a common complaint here.
1999 SLE K engine