might be power supply issue?
I would run at 800 and see if you get HW errors like that. With cpuminer, I have less HW errors on the blades than cgminer/bfgminer at any freq.
Looks like you are right. I'm using GS original industrial PSU. 1 PSU for 2 blades. With 800Mhz no HW. So i gurss i need more PSUs for higher fq.