1. it sounds like you glazed your original belt, this is caused by excessive belt slip, generally a riding condition
2. the machine creeping likely means your aftermarket belt is too wide or the clutch is sticking due to aforementioned glazing situation (excess heat), its putting burn marks in your belt resulting in the low speed jerking situation
3. The sliders being worm was likely caused by continued use with the glazed belt, the glazed/burnt spot in the belt causes the sliding half to not run true to the fixed half resulting in premature wear of the sliders