I have a ASUS 5870 and Powercolor 5870, but crossfirex fails.

if start something put load on the VGA's, the system reboot and get this error:

HyperTransport Sync Flood Error

how to fix it

system is:

Phenom II X4 965 BE @ 3400
ASUS M4A79T dlx -
2x 2048 Gskill Trident 2000
Corsair TX850