Toby

Couple of things I would check on.

1 - the trace going under the lower edge (going to R5) of the PCB. I do not know if it drives any heavy loads there.

2 - Vin to the MOSFETs - perhaps they are on the second layer and have thick tracks??

Jumper thick wires direct to the Vin lines till you find the cause. Jumper thick wires direct to the Ground points near the Mosfets till you find the cause. I suspect some heavy current switching on is ticking off your PIC to malfunction. I would typically see that the heavy currents flow first to the load switching devices(heavy loads) and then come back to the CPU(flea power)

Maybe someone else might see something I haven't caught