2005 Cadillac Deville OVERHEATING

I was loosing antifreeze around aug 09 and was told i have a head gasket problem. i bought some stop leak and put it in the radiator and didnt have any problems for 2 months then all of a sudden with no warnings it overheated on the hightway. i put on a new thermostat. didnt solve the problem. i took it to a mechanic and he changed the cooling sensor and took off the thermostat completly and it didnt work.. i reflushed the system using some prestone flush and it didnt work. is there a sensor i can look for besides the cooling sensor to change or is there something else i can do besides buy a new engine. i hooked it up to a diagnostic and there is no problems it says. the temperature on both radiator hoses was normal but was still overheating on the speedometer. i have no white smoke coming out of the tailpipes. there is no bubbles coming into the overflow tank. fans are working properly on radiator. waterpump is not leaking. help me please...