i made 12 in the winter 10 in summer on a 3.4 and stock belt, yours is not slipping unless your belt is shot.

if you really want a shorter belt, drive to a parts store and ask if you can test fit a few belts, figure out what stock is, try one a 1/2 inch shorter, most stores will let you do this. ive yet to find one that wont. most of the time they ask you leave your drivers lic for collateral. how i got my belt for the 3.0 and 3.2 pulleys.