HI can anyone help ?? my a3 is spluttering/ misfiring ?? between 2 & 3 thou revs, when you put your foot down its like it loses a bit of power then regains it constantly like a juddering effect ?? was fine the other day then put sum deisal in and jus started doin this dont no if sum **** in fuel but cant see it been this as it dont do it in first gear or when just revin it seems fine then, did fit a new ect sensor the other day but this shudnt effect it shud it ??