The temp sensor has two outputs and one input. This means taht one output feeds the cluster and another feeds the pcm. You've done all the right things with checking that the gauge or gauge feed is incorrect and the pcm sees it correctly.
This is sounding like a wire break etc. When the motor heats up the wire expands and pegs high. Try warming up the car and having a friend watch the gauge while you wiggle the wiring harness. Maybe you'll get lucky and it'll be the short leader that goes on to the sender.
