I'm having the same issue with the same equipment. Works for a while then they drop randomly. I have three nodes with Ubuntu 14.04 with 4x nVidia GTX Titan Black GPUs each. One may drop or all may drop. Blender is running fine on slaves when I look at them, and nothing in logs on Master that describes the reason. I've tried both 2.72. and 2.73 on both Master and Slave.