External Cache Failure reported and computer won't boot

Question

I have a highly upgraded Digital Audio G4 (originally a single processor 533 Mhz, now a Dual 1.2 Mhz). The machine has been running through many ups and downs hangning in there for a while. Basically everything has been upgraded except the motherboard. Normally, this machine runs like a champ and I changed nothing on it when it started behaving erratically. It just sat powered off for a while.
The latest problem ocurred out of the blue:
After being powered completely off for some hours, when I went to boot it back up, everything came on like it normally does (fans, power light, hd's sound like they're running etc.), but it would not boot. My monitor would just go into power save mode and display nothing. When the computer does this, I cannot reset PRAM nor can I boot into Open Firmware. There are no chimes and the PRAM does nothing. My keyboard does not respond to any external stimuli, it will not boot off the original CD. No keyboard input affects the way it boots. It is as though the computer powers on but starts up nothing.
The only thing I have been able to do to get it up and running is to hit the black PMU reset button on the motherboard. If I do this before powering on, then the machine boots up as normal. When I actually shut down and lose power to the motherboard, then later try to reboot, it goes through this process again and again - monitor goes into powersave mode, no PRAM reset abilities, nothing boots, sits powered on only, etc. If I open the case, hit the PMU button, close the case and then restart, the Mac behaves as normal... UNTIL you shut it down and try to power it up, then it goes through this process until the PMU is reset.
First thing I did was replace the battery with a fresh new one that I have. The computer behaves exactly the same. I have tried resetting PRAM when it boots back up, have tried resetting NVRAM when it boots back up. None of this changes the behavior.
I was at a loss with this and tried disconnecting my external devices which did not change the behavior.
The reason I now believe it has something to do with the External Cache is as follows:
I have two versions of Mac OSX on two different drives. In System Profiler, Panther told me nothing was unusual. All devices including RAM seem to be okay. When running System Profiler in Tiger, I found a new item in the list called "Diagnostics".
Under Diagnostics, there was this very perculiar message: "Power On Self Test - Result FAILED, Failure Type - External Cache".
What does this mean? Is there a way to fix this? I am sure that it's related to my problem, but what are viable solutions?
G4 Dual 1.2 / PB 1.5   Mac OS X (10.3.9)   running Tiger & Panther, 1.25 GB RAM

Answer 1

I have narrowed this down to an issue with my 3rd party Gigadesigns CPU upgrade. Though it worked flawlessly for about a year and a half, it now does not like being powered off. Works fine if you leave it on, but to reboot after a power down, the PMU must be reset. PIA! But at least it works. But it fails to get past POST (Power On Startup Tests) without the PMU.
By replacing my original 533Mhz CPU, the power off problem disappeared. Of course, that's not an optimal solution, so now I either keep the machine on all the time or it's PMU time.
Hope this helps someone down the road...