I just tried changing the BCLK a little bit ago and ran into a problem.

When I changed it to about 215 and upped the CPU voltage to about 1.55 my top most 5870 (which my Display port is usually plugged into) failed to respond..Wouldn't show the bios or anything. I had to plug the display into the bottom 5870 to get video.

When I went back to the bios and reset to the factory default I could then again plug my display into the top card...but I had to go back into the ATI CCC interface and re-enable Crossfire.

I can use the SINGLE bottom 5870 with a 4.6 ghz overclock without issue...but obviously I want Crossfire enabled.

When I try for really high speeds, something is messing with my PCI-E bus I'm assuming.

Any ideas?

Current Crossfire STABLE overclock settings:

AI overclock - manual
OC from CPU level up - auto
OC from Memory level up - auto
CPU Ratio setting - auto
CPU configuration auto
CPU Ratio Setting - auto
C1E Support - disable
Hardware prefetcher - enable
Adjacent Cache line prefetcher - enable
Intel Virtualization tech - disabled
CPU TM Function - disable
Execute Disabled bit - disabled
Intel HT Technology - disabled
Active Processor Cores - all
A20M - disabled
Intel Speedstep tech - enable
Intel Turbo Mode tech - enable
Intel C-STATE tech - disabled

BCLK frequency - 200
PCIE frequency - 100
DRAM frequency - DDR3 1601MHz
UCLK frequency - auto
QPI frequency - auto or
Dram timing control
1st iformation - 8-8-8-24 - 1T
2nd information - auto
3rd information - auto

EPU II phase control - full phase
Load-line calibration - enable
CPU differential amplitude - auto or manuel values
Extreme OV - disabled or enable

Current voltage 1.455 1.812 1.376
Cpu voltage - 1.44375
CPU PLL voltage - auto
QPI-Dram voltage - auto

Current voltage 1.131v - 1.508v - 1.111v - 1.508v
IOH voltage - auto
IOH PCIE voltage - auto
ICH voltage- auto
ICH PCIE voltage - auto

Current voltage 1.64-68
DRAM Bus voltage - 1.65
DRAM REF voltage - auto

Debug mode - string
Keyboard TeakIt comtrol -disabled

CPU spread spectrum - disabled
PCIE spectrum - disabled
CPU clock skew - auto
IOH clock skew - auto