MAX6675 and 50Hz noise
If your laptop is powered via a two-pin AC outlet plug then there will be no earth. In addition to there being no earth, the "0 volts" wire of the lap-top will be rising up and down to the tune of several volts p-p at the frequency of the laptop's charger. It, almost certainly will be a switching power supply running at about 100 kHz.

I would recommend that you do as you suggest but as a quick interim, connect a wire from the Arduino 0 volts to the body of the oven.