My A6 seems to have develloped an occasional cold starting problem, it starts, does a few revolutions and cuts out, the worst it has been it did it about 10 times in succession, normally it starts and splutters for a few seconds before clearing its self, it also does a fair amount of revolutions before it fires, it used to fire first turn, any ideas?? a mechanic friend suggested that its supermarket fuel to blame but I've been using Shell fuel the last two fills, is there a possibility of this, do I need to change the fuel filter?