Yeah sounds like your body needs servicing.

Problem when buying used backs is you do not know the history of the backs.
So same situation, you could get a back that will need servicing in a few months, or it could go for years.
You can hedge your bet by buying NEWER backs, if you can identify the approx age of the backs. I think you can ID the year of the CF lenses, but I do not know how to ID the year of the back.

Lenses also depends on what you have. The grease in the older C lenses are probably about 40 years old, and dry and gummy, CF lenses maybe 20 years old. Personally, if you have C lenses, I would replace the C lens with a CF, and hope for 10 more years of trouble free use. Then sell the C lens to offset the cost of the CF lens.