Lately, I'm running two on the USB HUB and I keep getting ZOMBIE on one of the sticks. It is not a specific stick, but one will usually ZOMBIE out while the other is working okay.
I currently have it running at 200 Mhz at 9.11 watts. It chugs along at 22 Gh/s and will just die. I tried giving more watts up to 10 W per stick, but that didn't help stability.
How're you measuring the current consumption of each stick?
Can you measure the voltage across the capacitor for each ASIC chip?
Finally, do you have a fan on the heatsinks?
In the time I've been playing with a 2Pac, the only zombie events I've had were when the cooling wasn't sufficient. Trying for minimum heatsink fan speed mine ran happily all day but then zombied about 3 hours after the building's aircon shut down for the evening. Running exactly the same setup with a higher fan speed and it's just run happily overnight @ 325 MHz with ASIC voltage of 0.73V. I feel it's worth monitoring both heatsink temp and ASIC temp on the back side of the PCB to get a feel for how effective your heat transfer is. Playing with tightness of the heatsink screws will let you find the optimum clamp force for heat transfer. Too tight and the PCB flexes and transfer is less good.