Now this is strange...
Does it stop working after a fixed time? Like 5 minutes, 10 minutes...
Then it might be some power-safe feature that needs to be shut off.
I have seen some similar situations in the past and the only solution was disabling all power-safe features in the BIOS.
<edit>
Try also to change the setting in /etc/lilo.conf to:
Sometimes switching acpi off completely helps...
</edit>