The engine keeps building heat so it may be the head gasket. also consider low fuel pressure which can cause a lean mixture in the cylinders which means higher combustion chamber temps. make sure the cooling fans are working properly Put a new thermostat back in and buy a new rad cap as well ( I find original parts work the best, not aftermarket ones) and run the engine - the cooling fans should start when the engine temp is around 94 degrees C and shut off at about 88 degrees C. also check the rad to make sure it isn't clogged - just remove the top and bottom hose and run a garden hose through the to to see if it flows quickly out the bottom - replace the rad if the level builds up and there is a slow drain out the bottom hole. Good luck.
well you may be wrong, some t/stats when closed stop coolant from going to the rad, and force the water to stay in the block, so the motor will warm up quicker, and when the t/stat opens it closes the by pass and forces the water thru the rad. now when there is no t/stat water can stay in the block or go thru the rad you can get hot pockets, put a new t/stat in and go from there
fan could be out, water pump could be bad, some of those cheapies have pumps with plastic impellers that can deteriorate and not move water ..