Randomly repeating key
I suspect this is a hardware issue and recommend taking it to an Apple service center.
This happened to me once on a Dell laptop. Our IT department replaced the keyboard and that fixed it.
To diagnose I determined it wasn't a software issue by noticing the key could repeat before the operating system had loaded. If you are dual booting Windows you could use the same technique to determine if it is a software or hardware issue.
Take it into your nearest Apple Store to get its hardware looked at properly. You seem to have ruled out potential software issues.