I'd be blaming the power supply. Disconnect say half of the boards from power and see if it becomes stable. The PSU in mine could not handle more than 4 at 1200MHz, and only 5 at 700MHz. I just strapped a second power supply to mine, each powering 3 boards at 1320MHz.
That is very well possible, my blade is using about 200 watts for the one 8 chip blade (218W in total). But running at 700MHz would draw about 115W per blade * 6 = 690W + fans so lets say 750W for the power supply. If it is a bad one it might cause it to be unstable.
(Running your rig overvolted at 1440 MHz it would draw 1500+Watts and doing ~132MH/s) <-- I do not recommend btw because of the power draw and the heat it produces, 1 blade like I have you can cool very well.