[steamship]

I think all the earth straps look like that. It's worth opening it up to see if the connectors are clean and free of any oxidation. The oil pressure switch shouldn't cause any oil spillage, unless they tried replacing it with the engine running.

I wonder if it would be worth checking out the ECU box in the plenum chamber, to see if there is any water ingress there, and if so, is there any oxidation on any of the ECU connectors. Although it doesn't seem to be a case of it not starting every time it rains or you wash the car, it would be something to rule out.

EDIT: Do you have VCDS or some other way of scanning the car for error codes? Don't know why I never asked that at the beginning, as there may be something obvious from the codes. If you do, scan the car, save the log file and clear all the codes. The next bit is awkward (since it's temperamental at starting), but you then need to take it for a drive and scan it again (and save) to see what codes reappear.