hey dude just a stab in the dark, but after I had my car tuned on premium 98 petrol I heard bp was the go, but I was to find out it was the worst. Wen ever the car had been driving for a bit and I gave it it would do what u were explaining, it would kinda stutta and back fire, felt like I had no power. As soon as I told my tuner I was using bp he said it was sh*t and to change to sell, ive never looked back and the problem was solved. so give that a go if your using bp