HELP!!!!
I am having the same problem - very evident in Centipede and Millipede. I have Hyperspin, MAMEUIFX 0.134, Core2Duo, 4gb RAM, GIGABYTE GA-G41M-ES2L LGA 775 Intel G41 Micro ATX Intel Motherboard, EVGA GeForce 8600 GTS SSC Edition 256MB 128-bit GDDR3 PCIe x16, SVideo to TV, IPAC, OptiPAC - both running over USB, 4095k swap file, Windows XP SP3.
The game starts great, then after about 2-3 levels, the trackball starts to lose responsiveness, the shooter starts to bounce all over the screen, then I lose responsiveness altogether. If I pause the game, wait a little, then play again, it's fine. Very weird. Seems like some sort of buffer issue.
Things I've tried to fix it that haven't worked:
Upgrade XP Pro to SP3
Change trackball from serial to USB (via OptiPAC removing power connector from IPAC)
Cranked down any accelerated BIOS settings
increased virtual memory swap file and made it fixed/custom at 4095.
Tried official MAME 0.134 build outside of Hyperspin
Tried MAMEUIFX 0.134 build outside of Hyperspin
Turned off joystick in mame.ini (I don't use one anyway)
Turned off artwork, bezels, etc.
I didn't have this problem in 0.90 or 0.128 - my last two cab builds of MAME. I have not yet tried an old backup version of these to see if they work now.
I'd be happy to post my mame.ini if that would help.