Strange and intermittent electrical problem

[Spiffle]

I would suspect a corroded connector block. You need to clean as many as you can find. It's a long and boring job but use some ACF-50 or similar stuff to do a good job.

Cable sleeves, electrical tape, strips and ACF50 and I'll make a day of it - needs a good tidying up anyways. I did poke around the fusebox and connectors before trying again though, and to my surprise there was no corrosion in the connectors I checked and all of them had clean metal contact surfaces.