Thermocouple Amp Fluctuations

I built a Cartesian dual head printer with AC bed. It has run for 6mths very well but recently my temperature values started becoming erratic when the hot end external mosfet cycles. I'm using a RUMBA+ board and power the thermocouple amps off the regulated 5V pins on the RUMBA. I replaced the thermo amps thinking they were shot but the problem persists with the new amps also. I've checked all the wiring and grounds but nothing is apparently wrong. Any ideas?

Thanks