On 7 Nov 2011 at 18:18, Pete Turnbull wrote:
  It looks very like the one I used to put my SGI Indy
keyboard (my
 favourite) on a PC, and I did very occasionally have that issue with
 one keyboard.  Another of the same model was OK, and after testing two
 or three I recall soldering a resistor onto one of the lines
 somewhere, but I don't remember if that was to fix the keybounce or
 something else - probably something else. 
You may want to try adding pullups for the clock and data lines out
of the keyboard.  4K7 should be just fine.
Just a hunch, nothing more.
--Chuck