i would like to add that i am having this issue too so it is not just the OP...

I have spent over 24 hours smashing my head against the wall trying to figure out why my sapphire 7950 4L simply will not mine, can not connect to servers, produce HW errors.

turns out i require 2 gpu threads to mine. 1 gpu thread will not connect me on to any server! i've been searching this issue and there are some people with this issue.

do note that i have 5870 and 5970 mining at 400 hash per core  and soon as i put a 7950 on to that machine h/w are produced!

i am thinking maybe it is either the brand new 5 * sapphire 7950 4L cards that have this issue or it is the operating system. hoping to get windows 8 installed and see what results i get.

current using server 2008 r2 and just like the OP too auto thread-concurrency give me 8960