We're back to hardware as the cause, the original diagnosis. Not what you wanted to hear, but I think every other possible source has been eliminated.
Just in case someone else needs help w/ this…
Maybe it's just me…but I think it's a software problem. I had this happen once when I tried modding some scrollbar files, and also once on a Hackintosh after installing some drivers.
Simplest solution: back stuff up via Finder or via a network device, and then use Disk Utility & your reinstall disc and wipe the drive.