[CaberTosser]

I’m betting that you’ve got aluminum wire, it oxidizes at the junctions and conductivity drops to hell. The connections can be redone provided there is enough wire because you’ll typically have to trim the aluminum back as you don’t want to re-do an existing twist because of how aluminum work-hardens and breaks. It’s also necessary to use ‘anti-oxidant’ on the exposed aluminum to prevent the corrosion from reoccurring, it’s available in little bottles and is a viscous liquid designed for the application.

Regarding the wire work-hardening, I’ve had many junctions with the stuff break right off when twisting on the marette (wire nut), it’s something you get a feel for but as a default I always strip the wire back for a fresh connection, the issue arises when you run out of wire to strip back in the junction box.

I tore out all of the aluminum wiring that was isolated to the basement in my old house, I was getting 88 volts. I still have to deal with it on occasion at work as it pertains to the heating equipment, pumps, etc that I work on.

Remember that your voltage drop may not be all at a single junction, you might be chasing the circuit back through numerous junction boxes and the connections within them. Main panel too. It may well be a job for a pro depending on your level of competency with that type of stuff.