Selectric trouble

I am not a selectric guru, but I have one and I think that the most likely cause is that the lubricant has dried out, isn't lubricating and in fact is acting like glue or adhesive. The "right" way to fix this is to have the whole mechanism immersed in a bath of solvent (quite a few things have to be disassembled and removed before this is done) to clean off the old lubricant, and then relubricated afterwards. This used to be common, but there are not many places now that still do it and it's expensive (probably a couple hundred dollars). I have a full IBM service manual for the Selectric II, which is virtually identical to your machine, but I only have it in hard copy, not PDF, and it's bound. Perhaps there is a PDF copy of this manual on the 'net somewhere. A guy who did the immersion cleaning used to advertise his service on E-Bay, you might to a search there, or a google search.