It is not as much the bios mod that makes your problems, more it is your configuration of the miner. You have to do some undervolting of your gpu clock and mem. If you do no undervolting, your cards need a lot of current! You can do undervolting directly with the miner software or with tools like OverdriveNTool, but you have to keep in mind, that if you run several tools which controll your gpu core and mem clock settings, they influence each other and that makes big problems!