would my ribbon risers work periodically and then not?
If you are using ribbon risers, stop wasting time and buy all new USB type risers. Ribbon risers are mechanically fastened to the end connectors.
Adaseb was being incredibly polite.
5 year old ribbon cable risers aren't worth crap.
Yes, I agreed. I would first suspect the ribbon riser issue. A new ribbon riser might create a lot of problem, and yet it is 5 years old.
Did you observe your GPU temperature and also physical equipment temperature, especially ribbon?
I 100% agree with the previous comments on this issue regarding of losing the ribbon cables.
As to your question, yes it is indeed possible and actually quite likely they did become brittle and the electrical pathway broke internal to the ribbon, meaning you wouldn't necessarily see the damage from the outside. The wire diameters used in ribbon cables are incredibly small, thus why most of them use two ribbons for the entire assembly. If the wire in one of the two ribbons broke this could very well cause the periodic works/don't work symptoms you describe.
A few years back I used to use ribbon too as at first that was the only choice we had, and even when USB risers first came out it was hit or miss on the quality. Newer risers have pretty much addressed most of those issues and functionality can now be taken for granted. Plus using risers often allows greater flexibility with GPU positioning than is possible with shorter ribbon cables.
In larger quantities (10+) they can usually be found for around $6-8 each, so while you may spend double that of a cheap ribbon cable, I think the benefits are clearly worth it.