a Quick test

Take the thermo out put it back together with out it and go on a drive like you did befor. If it still over heats on the freeway with the out side temp below 75 you have a possablity of about 3 things. 1.head gasket
2.pluged up fins in the rad /dirt or something blocking air flow on the out sid of the rad(blow dust out of it with air hose)3. fan clutch. good luck