Commodore VE SS Series II motor fault

I have a 2010 Holden Commodore VE SS Series II 6-speed manual that has done around 49,000 km. A miss has developed in the motor, sometimes it's at idle, at other times under acceleration. We have taken it back to the dealer's a number of times. On the last visit after having the car for four days they reset the throttle body, which seemed to work, but now after about five weeks and running on only BP Ultimate it is missing again. Why is it that a motor vehicle that is manufactured and designed to run on E85/E10 etc. is now strongly recommended to only use the most expensive fuel and only BP, but still have an engine miss. By the way it didn't show-up on diagnostics, but the technician drove the car for a few days and yes it missed for him. If you could shed any light on this problem it would be really appreciated as it's becoming extremely frustrating.