if the relay on the board is driving a contactor and the contactor is switching on and off there will be electrical arc between relay contacts .this causes electrical spikes in the power supply of microprocessor .so if u use a watchdog it will be worse becoz the watcdog will reset the micro everytime the power supply has spikes .to prevent this u must have used ground plane in the PCB.but now u can use RC filter parallel to the coil of contactors.use 1W 100 Ohm resistor in series with 220nF X2 capacitor and put this RC element parallel to the contactor coils.
good luck.