A common earth problem is the one from the Becm to the floor. As you look at the Becm fuse box its the black cable far left. It is about 9in long and comes loose on the floor just under the carpet.
As Fanatic mentioned above with the spray. You would be amazed by how many faults can be prevented by using something like WD40 every few months. I always spray it under the engine bay fuse box, back of the head lights, Rear lights, around the Coil Packs, Kick panels, center switch pack, Becm plugs and loom, under passenger seat.
You dont have to take anything apart and it makes the difference. Most of the failed P38s I see are down to Electrical faults with green pins.