Getting the "prohibitory" symbol on startup

I have a mid-2004 PowerMac G5 that until recently was running fine. It had OS-X 10.3.5 installed. At one point the screen froze, no mouse, no keyboard. I powered it down and back up again. That's when I got the prohibitory symbol on screen. - I have re-seated the hard drive cables, optical drive cables, the Video card, and the RAM chips. - I have reset the SMU and the PRAM (repeatedly). - I have removed the drives and re-formatted them on an external dock, they're fine. - It will not start in safe mode or recovery mode. - I was able to start it in Startup Manager, I ran the Apple Hardware Test (from the disc) and all the Hardware tested OK. I was able to start in Single-User mode, however it got stuck on "Still waiting for root device". - Last resort was to again format the drives, start in Startup Manager, and run the original installation disc. It got partway through installation and locked up again. It will no longer even start the install. This is where I ran out of talent. Any suggestions would be appreciated.