Has anyone heard of, or tried this:
http://www.marvelmysteryoil.com/index.php/site/mmo/
On some other forums folks talked about using that with great results (not in this particular vehicle), and it seems purpose built for fuel system cleaning and lubrication. Claims to be safe enough to add at every fill-up (4 oz per 10 gallons).
EDIT: Nevermind, I see Marvel was discussed a few pages ago. To the person that brought it up - did you every try it and if so what were the results? I may give it a shot out of curiosity...that is, after I try the Shell gas theory out.