Opaque window dragging under XFree86 4 causes sound to get jittery on an MGA g200. This is almost certainly the mga driver locking the PCI bus for imaginary performance gains in bus transfer speed. This wasn't reproducible under 3.3.6.
Changing esd priority to -20 doesn't make things any better.
This is a hardware problem with the MGA cards. Matrox lock the PCI bus when the
fifo is full, apparently as a speed hack.
XFree 4.0 has some aggressiveness controls that may help, and the 3.3.x server
doesnt generally show the problem