Problem with drives dropping back to PIO mode

AMD 1800XP, 512 meg PC2100 memory, Motherboard, I can't remember
offhand, but it's about 3 or 4 years old and flashed with the latest
BIOS. Two Maxtor ATA 80 gig drives on the primary IDE. DVD writer and
CD writer on secondary. XP Pro SP2.

The problem is with the primary drive dropping back to PIO mode in
Windows. I can't put my finger on when it happens exactly. Sometimes
it seems to be when moving data around on that drive. Another time I
was beginning to think it was only when I used the DVD writer, and
even bought a new one. Another time it happened when the computer was
left on overnight doing nothing. Today it happened when I was writing
v2 tags to 8 mp3's. Sometimes when I check the DMA mode, it has
dropped back to Ultra DMA Mode 4, or 3, or 2, or 1. This can happen to
both master and slave drives, though the slave has only dropped back
to PIO mode once.

I've fitted new IDE cables, tried a different memory stick, run
Maxtor's Powermax utility on the drives, which confirmed they were OK.
If I delete the MasterIdDataChecksum setting from the registry for the
primary IDE, and reboot, the drives are fine again for a while.

All this was beginning to seem like either a Windows or a HD problem,
until one day at boot up, drives began to be missing. Any combination
of master, slave and optical drives could be missing, though this only
happened over two days, whereas the DMA problem has been happening a
couple of weeks. The missing drives hasn't happened since fitting new
IDE cables.

The only hardware I am unable to substitute without buying new is the
motherboard/cpu, and PSU and HD's. I could possibly swap the drives
around, but not until I have backed up both 80 gig drives.
Unfortunately, the DMA problem means I have made about 15 coasters so
far.

Any thoughts on what might be at fault?