did you attempt a complete flush of teh system? its possible something is clogged up. If you have some time on your hands, remove the radiator hoses and pour clean water or coolant through the radiator. it should make an exit out teh bottom that is just as fast as you pouring it in. if its a lot slower, the radiator is clogged up. if its only slightly slower its probably just because of all the fins,core crap it has to flow through before it gets to the bottom.

if thats fine, you might wanna check the waterpump out. if it is not pumping the amount of coolant necessary to keep the system pressurized (or not at all) there is a "weeping" hole on the waterpump itself that leaks coolant. if it is indeed leaking while the motor is running, replacement is necessary, but only if you determine the radiator is perfectly fine.

although any thermostat works, the OEM ones last much longer and are much better quality.

if everything checks out and you continue to have overheating issues, bleed the system off all the air. the bleeder is jsut like the ones found on brakes. its mounted on teh front of the block right at where the upper hose connects. top off teh radiator, leave the cap off, start the motor, let it warm for a few minutes, then loosen the bleeder bolt a little. let it run (while keeping an eye on the temps, have a friend sit in the car) until only coolant is coming out of the bleeder


if that still doesn't work, post back.