Could you verify how the IRQs are shared?
I guess your graphic card is an AGP model.
If yes, do you have a sound card in the PCI slot that is closest to the AGP slot? If yes, you may try to move the PCI sound card to another slot. Sometimes, this slot shares IRQ with the AGP, and it is not working always fine.
In theory, IRQ should be shared, but some cards have trouble doing it correctly.
Something is wrong with your system, because with such a config as you have, the game should run as a breeze.