G4 500DP, OS 9.2.2.

Had a power outage the other day while working in Photoshop. Upon regaining power, the computer keeps restarting everytime I try to shut it down. Zapped PRAM, DiskWarrior etc..., but nothing's helping.

This happened once before when it was hooked up to a battery backup system; the machine was not on, but we lost power for a brief time. Upon regaining power, the machine mysteriously started back up on it's own. No problem since then until this problem now.

Please; any help?