Bizarre CD-Rom trouble

[pardsbane at offthehill.org (Josh Pollak)]

Ok, this is really puzzling me, and I'm wondering if anyone else has 
seen this. It happens on both Linux and Windows, so its probably a 
hardware issue, but I'm hoping someone can give me some tips anyway.

I've got a Shuttle K41G XPC, one of those small 8x8 cube PC's. The 
screwy thing is, no matter what CD rom drive I put in there, it refuses 
to eject. Generally, it will eject the first time I put the drive in 
the PC, but then it stops working. They will still read the CD's that 
are in the drives, but I can't swap them. When I use eject -t (or right 
click on the drive and select eject on windows), or if I just hit the 
eject button I get the same result:

The tray motor stutters a bit, the access light flashes for a minute, 
and then 'autoplay' re-engages, as if the drive thinks it just loaded a 
new CD, when in reality it just re-accessed the old one. Adding to the 
oddity, whenever the drive does this access dance, my UPS beeps like 
its being drained of power, or as if it has been disconnected from the 
wall (which it hasn't).

I've swapped drives repeatedly, and each time with the same result, it 
will eject and work fine once or twice, and then it starts freaking 
out. What could be causing this? I thought maybe the secondary IDE 
controller was screwed up, but it recognizes and reads from the drives 
just fine, so I'm really puzzled.

-Josh