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.