Has anyone had experience with this miner dropping hash rate significantly over time?

I have 2 rigs each with 13 x 1070s running on SMOS.  The rigs start out at a delicious 6300 sol/s, but over about 8 hours drop steadily to about 3500 sol/s.  

Obviously this is a non starter for me.  My only guess is perhaps the CPU is getting overwhelmed with so many cards in the rig?  I'm no Linux expert so I don't know how I'd even check that.