Sorry for the long delay in responding ClassicGMR! I have a solution to your problem! Basically there was a bug in the earlier firmware which has now been corrected. The bug set the joystick direction to the upper left corner after the first button was pressed on the gun. After you exited from the game it would cause the menu to scroll up forever until you unplug the gun. I got this information from the people at arcadeguns.com. Hope this helps! Oh, and the firmware fix can only be downloaded here for the time being (ie not from Ultimarc):
http://www.arcadeguns.com/download/Aimtrak.zip [version 1.8]
I didn't experience this issue before, because I was using a frontend with command line MAME. So, I decided to try using MAME32 and experience the same issue you were having. After updating the firmware this issue went away!! :) Hope this helps!