I tested with PC2700 DDR SDRAM @ 333Mhz the RAM type which officially supported by this machine.

I was using PC3200 @ 400Mhz

Same symptoms.

So the last thing to check is a Radeon card and / or driver.

Anyone with a spare Radeon card drop me a line :)

I am thinking to test a driver from an even older ISO -in the past I have checked an older one but can not remember now-, any recommendations?

Yeah, they aren't that picky about memory.
I've run PC2700 and PC3200 without a problem.
I may have a spare 8500 after I buy redrumloa's G4 this Thursday (if the 9600 I have on hand works in it).
Otherwise I think I have the flashed 9800 I was supposed to send you around here somewhere, but I don't recommend flashed cards anymore.
The have issues.

Thanks Jim. Check with the 8500 and come back to me. Other offers also welcome.
Yes I would also avoid flashed cards.

After vairous tests I have found the reason of this strange behavior -see my first post-.

After I deactivated the enhanced display option on the screen settings the machine works normal.

The G4 wears an ATI Radeon 9600 AGP. 256MB.

Do not know if it is a bug or something...

Did you still want the Radeon 8500 I received from redrumloa?
It works, but I replaced it with a 9600 I had on hand.
I'm willing to ship it to you at my expense since I owe you a favor.

Yes Jim! Will PM you about this! Thanks!

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Cool, glad to help.
That goes for anyone else outside the US as well.

Oh, and a quick note, red's G4 is running superbly, but doesn't liked to be overclocked (so eventually I'll be swapping in a 1.33GHz Xserve cpu board...as soon as I relocate it).
If you get the right Radeon 9600, they make a good match for the G4, and they are cheap.

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What problems are you seeing with your MDD overclocked, and is it the single G4 CPU module, or a dual G4 CPU module that you have overclocked? What overclock speed are/were you using?