PIO will kill performance. You definitly want DMA, getting it though can be tough.
I read somewhere that Windows XP will keep dropping the mode down when it thinks the optical drive experiences a read error. You may want to look this up in the Microsoft KB.
Also, sometimes optical drives just get bitchy and won't run in DMA mode if in a slave position or on the IDE chain with certain other devices.
You might want to try updating system drivers (like Via 4-in-1 if you have a Via based mobo for example.)