A friend is currently experiencing the same issue with a 3.5 in a '10 in Canada. Dealer has removed and cleaned his heads in the past, and is now about to replace them. With his, it starts at about 80 klms/50 mph. He is certain this is a symptom caused by the problem, and new heads will not correct the fault. Dealer is simply throwing parts at it. Becomming a serious safety issue and he is fed up. I'll keep you posted if a proper fix is found, please let me know if you get a satisfactory result.