I have a 12 xp900 and I ride mostly in the woods doing medium to big hill climbs. My machine seems to have poor power in high gear and 4x4. Almost like its not hooking up. I go to low gear and it seems fine. On moderate hills high should be just fine. Does it sound like a tranny problem ? The belt looks fine but feels like slipping when this happens. The stocker is a purple spring. Would a different spring help it hook faster and stronger?