"...problem of probe..." does not convey enough information, how did you "confirm" this? What type of fluctuations, large, small, speed of change, etc. ? What were you expecting for results and how did your observations vary? Has this test setup performed properly in the past? More info yields better answers.

"Problem of probe" --

1) under the same conditions, probe on other channel not mounted on coil shows right temperature.

2) When coil probe's channel is selected, either readout goes out of range or it shows any random highly fluctuating readings.

3) Expected reading was around 80 Deg C then.

4) Set up performs well with other smaller coils generating lower magnetic and electric fields.

You can try a grounded shleld of magnetic material on RTD.

Switch the HV off while measuring. If it's a big coil, being switched off for a few seconds every couple of minutes won't make any significant difference to your results (unless you're grossly overloading it).