Could the cables or contacts themselves be at fault? This isn’t my most knowledgeable area (most errors I blame on computer demons, tricksy little bastards), but with my extensive experience with my ageing Renault, most electrical problems, especially intermittent ones, are caused by bad contacts or leads.
Anyway back to me knowing what I’m talking about, I came across an excellent book in my library not too long ago, the Haynes manual of computer repair (I dunno if Haynes exists outside the UK, but over here its the definitive repair manual for any car, as well as a few other things). It gives you a pretty good understanding of computer repair, and takes you through quite a few procedures step by step