Hi Steve,
I have seen similar conditions in the past, This sounds to be a ground condition. Please ensure the TC is connected to ground at the signal source. Please ensure the furnace is connected to ground as well. In the past I have seen a similar condition with a TC in a motor case, after further examination there was a ground strap that was loose, causing the condition. It may be helpful to test with a different TC to confirm it is the field wiring and not the hardware.
Thank you