I've recently upgraded from an evga 8800gs to an HIS 4850 Iceq4 Turbo and am experiencing an unacceptable level of stuttering during video/audio playback. Prior to the nvidia --> ati switch, i experienced nothing like this, so i'm assuming it has to be the new card.
I did my best to wipe out all the old nvidia drivers first using the uninstall manager then by using guru3d's Driver Sweeper. I started with Catalyst 9.6 drivers, and just recently downgraded to Catalyst 9.5 drivers to no avail.
I'm also running the Combined Community Codec Pack (CCC) as well as ArcSoft HD playback software (which threw me for a look while trying to run Fallout 3)
I never notice any stuttering during gaming, only during video file / audio playback.
I would try to use ffdshow for video and audio filters. For Hd playpack I found a great boost in performance using ffdshow and setting the multithreading to 2 threads (because I have a dual core cpu). This utilizes both cores during video/audio decoding and playback.