Currently running 830/1250 so normal clock position on the cards.

Fractal Design Define R3   
AMD Phenom II X4 840 (4 x 3.2 GHz) AM3 2MB   
AMD Heatsink & Fan   
Corsair 2GB XMS3 PC3-10666 1333MHz (2x1GB)
2 x ATI Radeon HD 6990 - (XFX) CrossFireX Setup   
Asus Crosshair IV Extreme (AMD 890FX)   
Motherboard Integrated HD Sound   
Integrated Ethernet Lan (Broadband Ready)   
Cooler Master 1000W Silent Pro   
500 GB SATA-II HDD 7200 8MB   
Samsung (SH-D162D) 16x DVD-ROM - Black (SATA)   
Microsoft Windows 7 Professional 64 BIT

GPU-Z shots - Check out the shaders and texture fill rate???

Good Core:
http://img585.imageshack.us/img585/2937/ok6990core.gif

Problem Core:
http://img600.imageshack.us/img600/1500/problem6990core.gif

Check out 8 CU on first core when starting Diablominer..

http://img607.imageshack.us/img607/8104/8cu.jpg

Have tried with and without Crossfire enabled with a dummy plug.... no joy.

Thanks

D