Find yourself a known good case and transfer all the internals - at your leisure. If it fixes it then you know the case is an issue. If it doesn't then something internal that you brought from the infected unit.
That's what I finally did with my 727 that would back out of the shop in neutral. It was out three times and had three local transmission guys look at it, no one found anything. As a last resort I put everything in a different case, problem solved. I'm lucky this way.
