[xlr8or]

I had this same thing happen a while back and it was the head gasket that was the culprit. It was blocking the water return on one head (the one with the sensor in it) so there was no circulation on that side. If there is a spot in the other head to put the sensor?? or is it on the intake manifold?
If a head gasket were leaking exhaust into your water jackets the cooling system would be pressurizing as a result. Are you dumping water?? Also try running with no radiator cap and see if you are getting any bubbling at the top of the tank.

Had both happen before. Sure was pissed when I didn't see the difference in the one head gasket while assembling the motor.