That board has very light keys. Do you think it may be possible that you are accidentally pressing another key which cancels out the previous one? I don't know what kind of rollover the 89s has over PS/2 converted to USB. It could also not be rollover related, but related to the key assignments in the game (some action which stops you moving).
Try Aquakeytest and see how it behaves when pressing the other keys your fingers rest on while playing.
That's the only thing I can think of off the top of my head. Could be something else entirely, but Aquakeytest is a good tool for checking this kind of thing.