I've noticed my bikes, 07 450, power seem to slip occassionally in 4 and 5th gear. When I pin it on a strech on the paved road, around 2/3 of the way threw the power. Today, I went to give it gas to roost out of a mud pit section on a track and the bike didnt even spin the rear tire, I was a gear to high 3rd, clicked it down to 2 and it took off. Ussually it would just bog a little if that happend. Im not sure if my clutch plates are toast or not as it doesnt slip all the time. Should I just go buy new friction plates? What are some good ways to tell if your clutch needs new fibers?