well i have been using optimax for 2 weeks now and this problem has ocured in those weeks.
i have a Pilot Green Fuel saver fited to my car. Maybe because the pilot green improves the molecular combustion of petrol by means of magnetism and the optimax has additives in it, there might be some sort of conflict or sumthing.
i thing i will try going back to regular, see if that sorts things out!