PowerMac G5 1.8GHz OS Tiger won't sleep/immediately clicks awake

That's the long and short of it. Was working fine, going to sleep after 20 min. of non-use and waking up only when I rouse it. But starting a couple days ago, it goes to sleep at the 20-minute mark as it should, clicking to sleep then IMMEDIATELY clicking back awake with a WHIRR of the fan. Will continue to do this per its sleep schedule -- powering down then immediately clicking back awake -- and won't sleep after telling it to via the menu or command-option eject.

I've zapped PRAM, reset the cuda, did a round of Disk Warrior, too. Also tried dumping the com.apple.AutoWake.plist and com.apple.PowerManagement.plist preference files. Even gave it a fresh battery.

Still won't sleep.

Notes: Has 2GB RAM, is using all Mac peripherals (monitor, keyboard, mouse). Only things attached to the back is WACOM tablet (USB), Airport antenna, monitor and power cord. OS 10.4.11 (needs to stay this OS for sake of Classic environment and clients' needs)

Appreciate the help!