If you run the engine without the thermostat for a day you know that the rad is always getting coolant through it! this means that if you are moving (50mph plus) you will be getting lots of cooling (and I do mean lots). your engine should not be getting hot but just operating temp. (can take a little time) If this is the case all is well fit new thermostat. If not your will need to work out where the blockage is or if your water pump is not flowing the coolant properly!