I have the same problem too except my thermalstat didn't fcuk up, me and Troy (YLWGTR) change a new thermalstat this morning, we gave it a pressure test and by now we've at least flush the radiator around 3 times now and it still heat up.
On the way back from Troy place i stop by at a radiator place, ask them to clean my radiator out, the guy reckon my radiator cap screw up, gave me a new one. That help for a bit that i thought we have eliminate the problem but no, the car still heat up but SLOWER, take a while to get hot up now ( the new cap must have help a bit ) but still !!!!
The radiator place also reckon that i would need a new clutch fan cos ' when i was there, he tried to spin my fan ( car still hot ) and said it should not be spining cos' when car get to a certain temp the fan should lock up ( even when i've turned the inigtion off ?? ) but i did what WRXhoon said about test the fan see if it engage or not and mine did, it spin faster and make different sound...
Chances is my Fan do engage but not a %100?