If it is a new board, then presumably the relays are new, so it sounds like it could be a marginal pick up voltage, if the relays are picked up by a solid state device (most likely) then for some reason the circuit, or the power supply to the circuit, is not providing sufficient voltage/current.
There is a pick-up voltage and drop-out voltage for a DC relay, and the drop-out is usually much lower, once it has picked up it will stay picked up untill the voltage is quite a bit lower than the rated voltage.
I would measure the coil voltage when the relay acts up again and when it is working.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.