Markus says:

Default IRQ handlers don't work, if disabling interrupts, always mask out the 
respective IRQ enable bits.

For IRQs associated with certain bits in status registers, always clear these
bits before enabling the IRQ. Mostly, the IRQ is only triggered when the status
bits change. Therefore, always clear them at the end of an ISR, too.

The ROM OCIA interrupt is pretty obnoxious - not only does it control motors, 
but it also plays with the sound, too.
