http://mysewingmachineobsession.blog...ening-and.html
^ Scroll down a bit and there's a little writeup about lapping a 301's needle bar to take out the high spot.
eta: Look real closely at all of the stuff in the bobbin case to make sure there isn't something in there that thread can stick on. My rusty 404 was a bit "grabby" after I picked out all of the lint that had rusted to it.
I am practically no help, but I try anyway. I will run this question past my mother the retired home ec teacher and see if she has a technique for dealing with these Slants.