I would reset the CMOS (remove the battery for a while), backup all the data I need to keep on a separate drive and then erase the system drive (use a program like Killdisk to zero the drive). Only then I would reinstall the OS and see if it happens again.
If it is because of some latent virus (formatting does not erase the drive, only marks it as usable), this should take care of it.
If it is hardware, it will come back, and we'll have to think at possible causes of interference that is fooling the chipset into thinking it is receiving 6s from the KB. "Dirty" power lines, strong EM interference from nearby devices, etc.
Last resort, assume the interference is coming from the mobo itself (a bad capacitor that's not filtering noise, for instance) and replace the mobo.