I have noticed it with other games like Centipede, Asteroids, Tron, etc...
Sadly, I think it is part of MAME. I have tried tweaking the MAME settings, but nothing seems to help. I have used both USB and PS/2 type interfaces, both IPAC and Keywiz types... still no help.
Maybe someone can chime in and let us know if they have had any success??? Anybody???