I had the exact same problem with my G91. Meridian initially replaced the PSU but then when the problem recurred found a problem with the cable between the PSU and the power distribution board. I seem to remember that originally had two push on connectors but looking at it now, the non-PSU end is soldered in place.

The 5v current draw will be quite high so it's important there are low resistance connections. I think your search should centre around those wires from the PSU to the power distribution board.

Mark
MC200, AC200, M6, 557, G91