It's the joy of ownership
but seriously it could just be a loose connection . . . Electricity is a lazy thing, if it has to struggle it won't get out of bed for anything.... then sometimes it does...just for the hell of it.
I'd check all connections, undo them all,clean them up and apply a little copper grease and tighten them all up again. Pay particular attention to the engine earth, that bad boy can throw all sorts of things out of whack