Hmm.. 1351 mouse uses potX/potY, and they are "common" for port1 and port2. The game uses potX/Y for additional Fire on Joystick port 2. (Fire 2 = Jump/UP)
Is it a game problem or more hardware problem..
Maybe you can't have a mouse plugged in and use the program (driver) for the Joystick with three Fire buttons at the same time.
Maybe the Joyride program from CSDB is able to test this.