David,
I would say that you might need to upgrade the Drivers for your motherboard. I know that the Via Chipset has some type of glitch at times with the plextor drive. This is noted at:
http://www.plextor.com/english/suppo...enabledma.html
I have found that when updating all the drivers for your motherboard, alot of times this will fix any issues with the DMA. Let me know what you find.