You most likely need to replace the 3 tires in the cassette..
this issue tends to happen when the rollers are becoming worn.
there is 1 feed roller 1 lead pulley and 1 seperation roller that needs replaced...If you feel brave you can remove the unit that hold them to see how worn they are.. ( simply remove the cassette completeley out of the machine..on the left side holding the feed unit in place is 1 silver screw..remove that screw and you can pull the feed unit out of the machine...if the rollers look slick.. let me know I can give you part #'s of the rollers you need