Hrm.

So, I have a bitstream that will run error-free on the ztex board at 170mhz as long as I only use one of the three rings.  I can also run any one ring at 170mhz and the other two really slow (like 50mhz slow).  But if I use all three rings at full speed, I get errors all the way down to some pretty embarrassingly-poor hash rates.  I experienced a similar phenomenon on my own boards, but it wasn't nearly this severe and the optimal clock frequencies were still giving me 245+MH/s on my SG-2 boards (ztex uses faster SG-3 chips).

I'll be doing some more experiments on the clock-rate/error relationship this evening, but the important questions require a new build in order to answer, and that's going to take 24-48 hours (sorry, folks).  Still lots of tricks up my sleeve, but they take (build) time.