One hard disc attached to the motherboard's SATA port, and one DVD/CD
burner attached to the regular IDE port. Every so often, I check and
discover that the DVD drive's controller has "fallen back" from DMA
mode to PIO. I get it back to DMA mode by deleting that errant Primary
IDE Channel from Device Manager and letting Windows redetect it.
Eventually, though (days, weeks, I haven't figured the pattern out
yet), it lapses back to PIO.

Can anyone suggest what might be causing it and perhaps a cure?

- Samsung 18X DVD+-R DVD Burner IDE Model SH-S182M/BEBN
- Western Digital Caviar SE16 WD2500KS 250GB 7200 RPM SATA
- ASUS P5L-VM 1394 motherboard
- Windows XP Pro, SP2