I assume you have the newest version of the Game Device 1.5. Your problem,
I think, is caused by the sound card frequency. It is possible that the
sound frequency set in the configuration file of GP2 is too high enough to
interfere with your joystick.
I used to have the same problem with Flight Sim 98.
Try to look for a patch for GP2 and an updated driver for your sound card.
Or you can adjust the frequency in configuration file of GP2 (reducing the
quality of the stereo sound)
Good luck :-)
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