µC (Arduino) and Dealing with thermocouple noise
I suppose the motor is run from a VFD as well, which are notoriously noisy. 

Try adding a taste of resistance in the T/C leads, perhaps 100 ohms, and increase the datasheet capacitors by an order of magnitude (to 1uF X7R and 100nF X7R). 



Keep the resistors physically small so the temperature differential across the ends is minimal. Ideally you could graft a pair of smallish (0805 or 1206 is fine) SMT resistors between the terminal block and the chip. You can test it with leaded resistors hanging off the terminal block, but the temperature error will be larger than necessary. 

A grounded (at the PCB) shield on the thermocouple wires and any thermocouple extension leadwires may also help.