Problem in temperature transmitter because of VFD (Variable Frequency Drive)
If your sure you have proper grounding and shielding techniques then try a filter like this:





Source: AD8420 Datasheet

I would set the pole lower than 60Hz because in an area with AC mains (and higher frequencies from the VFD's) this would block out most of your noise. 

The board needs to be shielded also and the probe cable. Make sure the probe is shielded in a manner that prevents currents on the shield (tie it to ground only on one end, preferably the amplifier end).

Another thing is shielding can only block electric fields, not magnetic. If the sensor or amplifier is placed in close proximity to magnetic fields (or run next to cables with high currents in them) then that will most likely create noise.