I've been having issues with my 2002 Honda Civic Ex manual 2 door coupe 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 it when originally flushed out(radiator and thermostat done by a shop and a high pressure flush) its fully cleaned out now, 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?

 

It could very well be the water pump giving out. When you go over 3000 rpms it's pumping hard enough to get coolant to cool the engine however it could be a blockage too. You might need to pull the water pump, stick a hose in there, tape it up real good to seal it and then turn on the water and see if it's pushing hard out of the water pipe and the other areas of the engine.

You might want to do a compression test just to make sure the head is sealing and is not warped before starting anything though.