How's the main ground strap to the transmission?  I've the same problem,
wierd temps, I did swap in a new sensor, reads the same as the old! Strap
looks like a green caterpillar, frayed at the edges.  Remember the gauge is
simply a variable resistance, so any wiring resistance just gets added in.
I am going to run a real 8' long ground cable directly from battery - post
to the starter bolt on the transmission.  This should clean up that pesky
ground problem forever and put a spin back into the starter.  Will post
asap.  Tim