hi all I have a l322 3lt that has a sorta dead spot at 12000 revs it accelerates fine but then splutters if you hard accelerate it stops till revs drop down to 12000 again then starts spluttering again so to sum up below 12000 fine above 13000 fine the bit in the middle spluttering help plz sorry thats a diesel 3lt BMW engine 2005 plate