My initial assumption is your power supply its either faulty or lacking in juice, according to the GPU specs, the system must have at-least 600w PSU and since you are running 4 of them plus other hard-wares on he system plus you should have ample amount of headroom. Do the math and it would be most probably the PSU. try using A higher wattage PSU. Are there any other symptoms? does it properly shutdown or just shut off?