Keep on oiling and spend some time investigating all parts involved. Even machines we have had for years can have a missed spot. I remember a problem I had with my 99, it was the needle shaft. I hadn't spent enough time on the spot where the needle bar moves up and down in the cast iron part of the body. It took repeated applications of oil and running the machine for it too free up. The insident happened long after I first had it cleaned and sorted out. I don't know exactly what caused it, it happened after a flush out with a spray can oil and I suspect it was old dirt and grime gradually dissolving. If it's not flushed out quick enough in can turn sticky again. Some oils are better at this than others, but it's mainly about detecting oil points and getting oil where it needs to be and often enough.
Another spot often missed (for years) is behind the round plate in the back, make sure every moving joint where metal move against metal is well oiled. I know you have done this many times, but it still might be a minor thing like this. A good light is a must for these black cast iron machine, especially behind the face plate and the round plate in the back.
Still; ask your son if he can help you check the carbon brushes on the motor.
These are the most common size for the motor belts, both for Singer motors as well as Japanese. Pfaff motors are perhaps a bit larger. You can easily measure it.
I hope you find something for aging hands and joints. The right exercise and perhaps a clever diet supplement can work wonders. A difficult screw is just the right job to ask someone for a bit of help.