There are lots of great "trouble Shooting Hints" on the feed!!!
i REMEMBER WHEN I FIRST HAD a problem like this... I took apart the top of my machine. cleaned and oiled the spots, took the plate off and bobbin carriage and I was so shocked to see how the threads and lent collects there... a person I know who services machines said that each time you finish a project and before you start a new, clean out the feed dogs,,,using a clean narrow brush (like one uses for painting)about a half inch size) brush around the bobbin case, put in a new needle and fresh bobbin..
It works for me and never have a build up of lint..