i've been having issues with my car overheating over the past year. I replaced the thermostat three times, the radiator and then the sensor on the thermostat housing. the radiator had a ton of scale and some leak stop stuff in when originally flushed out(radiator and thermostat done by a shop and a high pressure flush), only sign of the head gasket is the overheating itself and when i go over 3000 rpm in almost any gear the heat on the thermostat gradually moves down to regular temperature. what are the best ways to diagnose this problem in a situation like this?