I don't know if this is what is biting you (or if this problem still exists), but there has been a long standing problem with keyboards where they don't do well at detecting events involving multiple keys. Traditionally, all of the keys are scanned as being at the intersection of a small number of wires coming from the encoder chip rather than having one wire go to each key from the encoder chip. Shift and control get their own dedicated leads to the controller chip because they're expected to be used in combination with other keys. So, you might try using control instead of the space bar.
I have one of these babies: Classic USB NES Controller for PC
. That's another way to avoid that problem.