OK, let's try some other possibilities.
One thing that comes to mind is the internal backup (PRAM) battery. A dead or weak battery can produce weird symptoms including the inability to boot. You can get the battery at
Other World Computing and, if you need instructions, view
this video. Radio Shack may have the battery in stock. It will cost more than the $5 at OWC but you won't pay shipping. The battery lasts 3-5 years but will weaken faster if the computer is unplugged for periods of time. When I had a G4, it would do a partial boot and then shut down. Replacing the battery cured it.
Another thing to try is the procedures in
this article.
Did you try resetting PRAM and/or resetting PMU? The links that I provided have the complete instructions.
