At the recent school at Nelson, I encountered a puzzling problem that I do not know where to begin to start looking. In my A1 Scirocco on the next to the last session on the 1st day. I spun in 13 and ran through some fairly bumpy ground on my way to the front straight. After the session, I came in and got about 5 gal of gas off my neighbor in the paddock (who had blown up and was heading home). On the next session, in the "Oak Tree" and 13 both I suffered heavy backfiring and sputtering (the car acted as if it was starving-even though I had about 2/3 of tank full. The next AM I purchased fresh (92 Octane) gas, and topped off. I still had the same problems at Oak Tree, 13 and by the end of the day in 5. My instructor (who has a lot seat time in VW's at Nelson) had never seen this happen. I have narrowed it down to either I raised up some tank sediment spinning in 13 or bad gas. What do I need to check first? (The pump checks out already) Should I check the fuel pressure next, or get the tank flushed? Has anyone had a similiar problem?