[G4] MDD no start, grey screen, no bong

[Technophobic_Tom at comcast.net]

Ran and booted for months with no problem when restarted or after 
shutdown but on coming back from sleep-mode was dicey. Last 
productive days it would startup on maybe one out of four attempts 
after sleep. When it didn't restart after sleep it would only respond 
to the power button.

 From a dead start; push the power button, the button light comes on 
doesn't go off until the button is released, the fan runs quietly, I 
hear/feel the HD or DVD index two or three times (I think that's what 
it is), after 8-10 seconds the fan goes into turbine mode and that's 
the way things will remain - no bong. I push and hold the power 
button again at which time the button light comes on and remains on 
as long as I continue to depress the button. After about 5-8 seconds 
the machine shuts off. The button light goes out after I release the 
button.

I've reseated the graphics card, the memory, and all internal cable 
connections. I've tried every possible combination of PMU and battery 
antics that I've seen on the forums and that I can matrix. Yes, the 
battery is between the specs of 3.3 to 3.6v -- 3.548. Power supply 
provides 5v and 12v at HD connector, 5v trickle when powered off, and 
25v. Not sure about the 3.3v lines because I'm not able to easily get 
to them with the PS powered on.

I have some concern in that I've read where the PMU should only be 
pressed once and then not pressed again until the computer has gone 
through a power cycle. Does the fact that my machine has not fully 
restarted constitute a violation of that rule and what are the 
consequences? However, the same failed attempts were experienced 
repeatedly even before the case was opened.

Tried booting from the DVD. Tried booting with either HD 
disconnected. Tried booting with all drives removed. Of course, none 
this matters because the boot never gets past the POST (BONG! 
hardware check complete).

One possibility I've considered is the front panel board (FPB). The 
power button requires considerable force but unless there is more 
than one contact in there, the switch apparently is working. The 
other likely, and more expensive, possibility is the 
logic/motherboard.

Have I overlooked anything? Help!

Specs: MDD dual 1.25 pre-2003. Two sticks of 512 PC2700, basic nVidia 
GeForce4 MX with 32MB, SuperDrive, Apple Pro keyboard, Logitech 
Marble mouse, two 160 ATA HDs with 10.4.9 and 9.2.2 on both. Just a 
vanilla G4.