You can be sure it's something in the mechanism. You don't need to take these models fully appart, and the closest I know to a full disassembly is a tutorial for model 66.
Poke down the oil points on top of the machine, there are at least four holes there and check if they are clogged up. The upper shaft turning in there might need more oil. Oil in excess and turn the hand wheel.
Have you double and tripple checked oil points, you have detected them all? I wouldn't even bother thinking about further disassembly until day 4 of oiling and test sewing. Hardened, sticky oil any where can cause this. Grime and much stuck where the needle bar moves up and down cuases this. This is spot on what we get when we take out a machine that's been in storage, and we always start worrying it's something more serious. I have used spray can oil to flush out sticky machines, but you quickly have to turn to regular oil, and keep on oiling a bit the next few days.