So i got an MSI 7950 a couple of weeks ago but have only just managed to put it in my computer because of some parts that werent delivered.
I am not watching videos at all and when i play games it never moves off 500mhz core clock and 150mhz memory clock.
When i tried to overclock it nothing happens.
It says it has changed but on MSI afterburner the bit where you monitor it stays the same mhz.
Btw an edit, googled quickly and are you leaving browser windows open after watching videos? Some ppl reported issues there, also they were saying msi afterburner seemed to have some issues so try uninstalling all drivers download the newest directly from AMD and see if things improve.
i had this same issue with my 5870m when i downgraded to windows 8 from windows 7. my drivers were one version out of date, so i updated them and it didn't help. i then reverted my drivers back 3 or so versions to 12.8 and it works fine now. i dont care too much for using outdated drivers but as long as the clocks are right and i can overclock i'm happy. i haven't tried the new 13.1's yet though.
edit: msi afterburner also had said it wasn't changing the clocks, but catalyst did. it was not changing the clocks though. once i used the older drivers everything would register and afterburner works fine now. not sure if all this is the same as your issue, but it sounds similar. also i dont know what OS you're on. i did not have any issues on any drivers with windows 7. issues only cropped up with 8.