Gosh, bkay, no luck yet? Two thoughts come to mind: needle bar and lower mechanism. (Well, three, a bent shaft from the handwheel. Let's ignore #3.)
Heat the needle bar area with a hair dryer. Use WD40, or electro motive cleaner, sprayed on the needle bar, to cut through any possible gummed up old oil. (Cover the bed with a cloth). Charcoal lighter fluid can work, too, applied with the tip of a screwdriver blade. Turn that handwheel several times to distribute the "solvent." If it loosens, you've found the source of the issue. Repeat, then give it a good chaser with fresh sewing machine oil.
You can do similar with the lower mechanism, but there are so many moving parts. Focus on the "below the bobbin area."
Speaking of bobbin area, have you removed the "case" that holds the bobbin in the race, and thoroughly cleaned under the rim? It is amazing how much crud gets stuck there. I have had more than a few machines with enough bobbin case crud to freeze them up.
I hope this helps. Let us know how it goes.