This can be a sticky problem when the codes don't mean a thing. I have been dealing with this in the jag and my cargo vans. Nasty misfires at highway speeds. The solution is quite simple but undetectable, a "ground" wire has corroded! Those tech's love to replace parts don't they, coils etc. Quick fix. love the energy!

 

Despite what you post, the workshop manual etc includes wiring tests - right?

(However, on these cars in actuality it's hardly ever the wiring.)