Yes, exactly. heatsinks have been CNC'd and processors were fitted on them. Everything was perfectly running, very good temps also. Until one day, I fired it up and found this kind of desktop corruption you find in the enclosed photo. It's not the GPU, I replaced it with two different Geforce 6600 from other (functioning) G5s and the issue was there just the same.
Wait, from this picture, I can only see a single 2.5GHz CPU. Are both of them installed?
I would not use a modded OS, as long as the root cause is found at least.
So first let's check if the CPU cards are socketed, and disconnect any optional add on cards. Connect only the disk that you use for the OS. Reset PRAM and NVRAM. Then run thermal calibration and the extended test in Apple Hardware Test.
Since you tried known good GPUs, I have a feeling one of your RAM sticks have failed. I have also encountered graphical glitches when using a cheap SSD with a SATA addon card in my G4, so it may even be that your hard drive is going bad and the OS is corrupted.
So if you go through all the steps and AHT doesn't report RAM issues, it may find the hard drive is failing.
If not, I'd reinstall the official Leopard and then run some tests on that, to stress the system and see how it is under load.
When does this glitch appear by the way? What do you have to do to trigger it?
Note:
My G4 MDD is picky regarding RAM, so when I was debugging it, the tests would in some cases finish and report the RAM issues only when I ran the extended test. In some cases AHT would not even load but freeze, or freeze while running the memory tests. So I recommend you take out all modules and use only one pair that you know are working and compatible, especially if you're using different modules. This way the tests will complete faster too, and you'll have time to check the rest afterwards. I have 8x2GB ECC DDR2 sticks in my Quad and the extended test took around 10 hours to finish.
