Sounds like grounding, especially given that it works fine with the parallel instrument.
When you say parallel are you talking about physically parallel (two TC, two electronics)
or electrically parallel (one TC, two electronics)?
Are the TC shields grounded only at the Fieldpoint terminal block? Is the Fieldpoint
well grounded? Is the furnace well grounded?
Do the readings become erratic around the target temperature of the oven, i.e.
when a controller starts cycling the heating elements off and on?
Sorry this is mostly questions but maybe there is an AHA! lurking somewhere.
Matt