MDD DP G4 elusive diagnosis - freezes during boot up. Tired- HELP!

Question

After not using my Dual Bootable  DP 1.25 GHz MDD G4 for a couple of months, it's now freezing during the boot process.
What's weird is that it boots in Target Mode just fine, and when connected in this mode to my G5 Quad, it actually boots the G5 Quad in Target Mode just fine!    The G4 drive shows in fact as the boot drive on the G5 Quad's monitors.
I was able to boot off a retail Tiger 10.4.6 disk, so I did an erase and install of a fresh Tiger OS 10.4.6, which I promptly updated to 10.4.11.  After trying and failing umpteen times to boot off the HD, even in safe mode, I ended up being utterly unable to get either of the Super Drives to open normally, so I had to resort to the straightened paper clip trick to retrieve the install DVD.
Now even the paper clip fails to open either of the optical drives.
I have twice wiped the HD from the G5 Quad after booting up in Target Mode, and installed fresh copies of Tiger 10.4.11 as described, with the Tiger install DVD in the SuperDrive of the G5 Quad.
Since in Target Mode the G5 Quad boots just fine of the G4 HD, I think I can rule out the Hard Drive.
I think I can rule out the battery too, as it boots fine in Target Mode and in the regular boot process it sometimes gets as far as playing the OS fresh install music after going through the registration process.
I do have the original install disks with AHT 2.0.2 that came with the G4, but I can't get the optical drives open to insert the disk in either one of them.
I'm thinking it could be bad RAM, but why would the optical drives be stuck, both of them??!
I'd be most grateful for any tips as to what to try next.

Answer 1

Jump all hard drives Cable Select and place the intended Master on the end connector of each IDE ribbon (each bus).
Can you clarify, please?  Are you referencing the actual CPU?
Yes. A very fine layer of corrosion on one or more of the 300 pins could render the machine incapable of achieving logic.
The step should be performed after doing all the other reseats that I suggested.
I've never re-seated one,
Not so difficult.
Replacement of the thermal compound is a requisite part of the proceedure, however.
Use Arctic Cooling MX-4.
No worries if a bit too much is applied, unlike silver based compounds....... bzzzZZTT! Dead CPU.
My tutorial:
9AE7FE0E-0CF2-4A7C-8003-489B282582BC.html
I posted this for someone who wanted to remove a MDD logic board (edited for clarity, I hope):
To remove the logic board (of a MDD):
-Disconnect all cables connected to the logic board.
-Remove any PCI cards, and RAM.
-Find and remove the five screws that hold the heatsink. Remove the heatsink.
-Remove the processor board by lifting the edges of the daughter card. After successfully pulling the card free from the connector, it might help to slide the processor board a bit to the rear and/while lifting out, being careful to clear the two mounting pegs.
-Find and remove the one screw that holds the logic board.
-Slide the logic board away from the back panel, lifting to avoid the chassis hooks.
You need to pay attention to the three steps in bold. There is no screw retaining the CPU daughter card. It is sandwiched between the heatsink and the logic board socket.