Fortunately the feedback and the crackling problems are over finally. Also, my PCM91 is working great using the AES/EBU IO, i guess using the IO's on the "Main board" solved the crackling of my synths. I also put my 3 scope cards in the middle pci slots, and some changing of the board-id's in the cset.ini solved also the very annoying feedback.
The only problem i have right now are the " communication errors " with board 2 (which is my latest installed scope pro type II card). At asks to reload dsp's and if i press OK, CPU increases to 100% , and it nevers stops loading. If i quit scope using taskbar, it crashes, and i have to reboot my pc. No matter what sequence i use in the cset.ini file, it always points out to this board...
So, i took this board alone, and did a fresh scope install, and copy my backup on top of this. However, when i use it "stand-alone", no problems occur whatsoever. I can load all devices till dsp meter shows red. When i connect another scope board(s), it shows these " communication error " randomly. Sometimes when increasing/decreasing poly, or changing presets, or sometimes loading the project. This is the 3rd board in my sequence. When i create a project with 45 dsp's, problems occur when dsp meter is around 2/3 , where my 3rd scope card begins.... Mostly not immediately, but after restarting the project, or adding other devices.
The other scope boards work great together, but this one only likes to play alone

I already did a few "re-seating" of the cards, and cleaned the gold plates thoroughly with deoxit 5, so im pretty sure this is not the problem. Cleaned/changed my STDM cables. I think i also did most of the known tweaks, like irq sharing/ usb off / bios tweaks / cset.ini edits.....
Only thing i can think of right now is changing to the original IO plate of this board ( replaced the original classic to a AES/EBU ) , and see if this makes any difference. Otherwise a hardware failure maybe??
Anyone having any experience like this?? What number of STDM's is recommended, and to which connections??
Any ideas are welcome
Cheers!!