zapping the PRAM and VRAM did nothing for you, it's probably time to consider an Archive and Install of the OS.
All you need is your 10.6 installation disk, boot from it and go from there.
Archive and Install (preserve user settings) is the default install for SL and will keep your other data intact
I have an Intel Imac G5 2.66 gHz 2 core duo running 10.6.3. I am unable to get it shut down or sleep. Choosing either results in the display blacking out, the fans run for about 1 second and then everything come back on as if nothing has happened. I've reset the Pram and Vram, disconnected all USB and Firewire devices, tried it with and without file sharing and Airport operating. A 'hard' shutdown with the power button works fine. I have checked for any firmware updates but mine is fine. This happened once before and clearing out some files from deleted apps cured the problem but it's back now. Any ideas please?