FYI-Been dealing with plugin connectors on boards for years. Re-plugging fixes 90% of the problems. Back in the 80's I asked an engineer for TRW what causes it. He said it has to do with dissimilar metals. The board connectors are usually gold whereas the ribbon cables and other cables are cadmium-plated. A small amount of corrosion appears over time and messes it up. Solution: Replug on occasion with the power off, or get similar metals for the connection. Gold to Gold or cadmium to cadmium.
Cheers!