A lot of system memory is not required for cgminer.
could it be your lookup gap went to 1 ?
(it basically use 2x the amount of video memory compared to lookup-gap 2 ... but at thread-concurrency arround 20k, it does not use your 3GB of memory anyway)
Or do you have some other video card in your new PC which could be used instead of your 7950 ? (even an integrated one)
Also, no other instance of cgminer running in the background ? (happened to me ^^)
There is not another instance of cgminer in the backgroud lol. I also thought maybe it's about drivers...I tried few versions with no effect.
I tried increasing lookup gap to 3-4 and I can set higher concurrency but my hashrate never goes over 350kH/s.
I have integrated graphics so I'll try to enable it maybe it helps