You have 6 gpus and 1200 watt psu. Although you have tried other bioses, have you tried testing your system by connecting 5 gpu at a time? You say "you can only mine for a few moments and system crash", with stock clocks/no mods: Each gpu may pull around 200 watts from wall, your system should use 60 watts at best bet, therefore it might be a power resource issue. If you have a power supply with more watts, you may want to test your system with that or make sure that you modify your gpus correctly.
Hi! Yes, I have been thinking about that, I have already mined with 5 to 6 unmodded gpus, consuming around 100-115 watts per card, and everything seemed to be working fine, anyway, the main problem comes in when I mod the graphic cards and try to get them all to work, thats when I start losing access to bios, constant restarts, during windows loading, during bios, mining, etc.
Yesterday I tried a pair of sapphires nitro+ 570 8gb that went from 21 to 24 mhs after mod (I was soo disappointed), but somehow looks like they all are damaged, tried them on this different system and it allowed me to enter bios but i could never load windows because of restarts.
I also tried resetting bios aand reflashing it, and it only allowed me to enter bios with any different card than the used on the rig.
Soo many more random issues than the Rx 4xx's

I used to build 2 and 3 rigs a day, now I get stuck in the same rig for days! I know that's not normal at all...