May I be so bold as to point out one other thing that has been suggested that, as far as I can tell from the various posts, has not been checked out? That is the possibility of contaminated fuel. The purge treatment may give some insight on that or running the engine on fuel known to be good while the hoses are set up for the treatment.
Another of my $.02 worth.
Larry, for goodness sakes, get this solved! We are all going nuts trying to figure out what's wrong.