...That suggests that you maybe ought to check, i.e. swap-out, your cables.
FWIW, there still has been no more screen flickering since disconnecting the Ethernet and FW800 cables from the iMac and restarting. Network and external drive continue to function normally. Go figure. I spent $96.00 on Apple FW800/800 cables for the new OWC external drives located on the iMac and Mac Pro. They have not yet arrived. When they are installed, I hope I feel $96.00 worth of increased reassurance that a potential vulnerability in the system has been eliminated.<grin> Truthfully, I've become something of a fan of higher-quality cables. I was disappointed that I couldn't find a Monster cable that was FW800/800.
