|key is literally stuck (in pressed, or "down" state), or kbd|
cable is bad (internal short across data/clock/ground/vcc)
not to be obvious and/or condescending (i never mean to
imply you haven't thought of/tried) bum a keyboard and try.
if still problem, unhook kbd and use only mouse. If still
problem, its on motherboard. best I can feebly offer...