acpiconf -s4 cannot wake up: ACPI bug, kernel bug, BIOS bug?

Hi,

I have a i386 machine with Gigabyte 7ZXR mainboard here (Duron, VIA
KT133 chipset, AMI BIOS).

acpiconf -s1 and -s5 are apparently "working" ok, -s2 is refused (OK),
-s3 is refused (might be I didn't set the jumper correctly :o) and -s4
puts the machine to some sort of sleep (power LED blinks, screen blanks,
ATA hard drives park), but it doesn't properly wake up from S4. It tries
to, spins the disks up, I can see a screen, but cannot type anything;
the machine goes back into s4 right away, and is in a permanent loop,
lay down to S4-sleep and wake up. Pressing NUM LOCK doesn't toggle the
NUM LOCK LED. X11 hasn't been started, just text console mode.

Any ideas, or questions for more detail?

-- 
Matthias Andree