Your power supplies are too weak, as grumpy said, are you sure they are 60+ amp single rail.  I am having this problem with some of the Chinese factory power supplies!  They built these units with the bare minimum components to run, saving all expenses and sparing none!  That is the reason for the warnings not to over clock and run beyond the factory settings of 1300 mhz.  These units were tested for 10 minutes or so on the factory power supplies and the called it good.  The factory power supplies can not hold up under the long run times and heat, it is a marginal power supply for these miners,  So may be good enough, but all are not!

As I wait for ally upgraded power supplies to arrive I have my miner under clocked to 1100 mhz, this leaves enough power to go around to the whole system.  It also keeps the running producing around 150 gh/s better than no hashing!

I have purchased and installed a pair of Corsiar CX750 in a unit, it stopped doing this and is running great!  I have two other types of power supplies arriving Monday and Tuesday.  I will let everyone know how these work.

Mtnminer