Hi, thanks for your reply and good thermocouple explanation!
but i don't think the cast iron firebox is grounded? it is non-electrical...
however, i did an experiment and removed the stainless steel sheath from the thermocouple so it is now just two wires covered with fiberglass sheaths.
the probe is now inside the fire box and reading fairly reliable, so for some reason the metal braided sheath was causing the issue?
problem now is how long the exposed wires will last in such a harsh environment! im going to try and find some better fiberglass type sheathing i guess...