FP TC-120 readings freeze after about 30 minutes

[TRCErin]

I have a very simple VI that controls heaters in a chunk of steel.   The steel has four thermocouples(one for control and three for monitor).

The VI loops and looks at the setpoint and the control TC reading.  A boolean determines if heater is turned off and on.

Everything runs great for awhile (usually less than an hour) and then all the TC's "freeze" into a reading that is just slightly higher than the setpoint.  Consequently the heater stays off and everything cools to room temperature.

I'm using a TB-3 and V/C are not hooked up.   This problem happens if one thermocouple or all of them are hooked up, it makes no difference.

I'm at a loss for ideas.  I do very similar things with other TC-120s at my facility, but I can't swap them since they are all currently in use for testing.

Any ideas?

[Tom_W_[DE]]

For anyone watching, it seems that TRCErin has worked out the problem according to the results in this other thread.

Thanks-