About one year ago (December 2011) I got some bad fuel in Ticonderoga, NY. Immediately after fueling up the truck started running very rough. I quickly got a bottle of 9-1-1 and put it in the tank. When I got back home I changed the fuel filter (no water observed) I ran all the fuel I could out of the truck and then filled up with fuel again and put in more 9-1-1. It has never completely cleared up, occasionally at idle in gear it will run rough it will also knock slightly while under a light load. Now it is starting to "stumble" just a bit while under load but not all of the time. What does it sound Like to you guys?