I have a '99 volvo s80 t6 as the title says and it has some serious issues. It is mis-firing on two cylinders, which is causing some serious problems. Barely wants to idle and etc. was just curious as to where I should start looking for this problem. Was going to change all the plugs as they look really bad, the oil and filter as it is black but after that not sure how to diagnose what is causing this without paying a shop several hundred dollars.