Oops, I forgot to remove the 'jump' button. The problem might be you are pressing it, witch will just make you walk up. B1 is the broken jump button, B2 is the actual fire button. Though that shouldn't do what you describe. Try pressing A to toggle off and on the ultra-buggy Analog Aiming Mode and see what that does. It works fine for me.
Edit: New version now. Removes pesky fake jump button, and sets B1 to fire. Still cannot reproduce your bug. But I did notice things were really screwy when ZSNESW was loaded in the background. Press D and see what the frame time is. If it is ever 0, or over 3 or 4, that may be the problem. Close some programs and see if that helps.
Edit edit: Also, Nevermind about Dezaemon. It has nothing to do with Dezaemon anymore. It just did back when it was a space shooter project. Now it has more to do with the surprisingly good Gameboy Color version of Daikatana (which is freely available
here). It's actually good! I swear! Like Final Fantasy Adventure, Zelda, and Crystalis all rolled up into one.