I have this problem and i narrowed it down to the spark plugs.

i was speaking to a supplier of denzo iridium plugs not so long back, and told i had this problem. he told me any car that is running 20% more power than standard is obviously going to run hotter so 'colder' rated plugs are needed. i have got ngk plugs in mine and they need replacing every 4-5 months.

the denzo plugs cost £52 but if you get the right 'coldness' setting for the car they will last for 70,000 miles.

might be worth checking yours havent cracked. they somtimes crack inside so is unnoticable.