I would say you can forget about ACPI power management on a machine that old, not that old machines are bad, just that APM was the PM scheme when this laptop was made. I suggest you disable both settings in BIOS and append LILO for APM power management. I recall that the actual appending code included turning APM on and as well as turning it off. I'll have to crank up my test box, see what's in LILO, if I can't find it on the VL Forum.
You may have to append in more than one section of LILO, but especially, the append should be in the global section, IIRC. (All this from my memory, apologies.)
If this fails, try enabling AC PM in BIOS (again). Other combinations, too. Don't give up. Give things a reboot to take effect. Keep an eye or ear on the CPU fan, make sure it doesnt cut off during boot.