@bkay by the sounds of it, it is more gummed up than you think (in the the hard to reach areas). The feed fork is the rod that the stitch length lever connects to inside the pillar and the slide block is the piece that allows the lever to move correctly (the block issues usually cause either a sticking lever or a loose lever but gets gummed up also with sitting for long periods). From the feed fork, check all points along to the feed dogs, adjust, clean and lube as needed (i would be surprised is something is worn). Then adjust the pressure on the foot and do a test, hopefully that will help
