once a belt starts to slip a little they can get glazed on the sides. clean the clutch faces w/ a scotch brite pad, and if you want to milk the belt, lightly scuff sand the sides. Best bet is to change the belt. It does sound like your getting snow in there some how. After you notice it slipping, open it up and see if it is wet by the clutches, from melted snow. This is part of the reason you shouldnt do water crossings, if the clutches get wet, you sink!
