I would not give up on first or even second attempt. My suggestion would be to keep on cleaning and oiling, checking the meshing gears and worm gare carefully when you turn the wheel by hand. Go over every oil point, a drop or two with oil, don't forget the needle bar. Take out the bobbin case and as many parts that's needed for the basic maintanace and oiling, take off the throat plate and inspect moving parts carefully. What you describe is quite common and with a bit of persistance you will get to the bottom of it; more oil to free gummed up parts, dry parts,... Further cleaning and poking might discover a tip of abroken needle stuck some where, a bit of thread,...
There are still some clever guys posting here on QB, quite a few have the 301 as their favorite and
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