I just bought an ASUS HD6670 the other day, replacing my long time GeForce 9600GSO. I uninstalled the drivers for the 9600, and installed the current up-to-date drivers for the 6670. I use my PC for lots of video playback, and I do it all through Windows Media Center. Now, during all playback in WMC (including Live TV, Recorded TV, and MKVs played using MediaBrowser) there is a slight stutter, or what looks like a frame drop every few seconds. It is incredibly annoying, and I never had this issue while using the nVidia card. I have an Athlon II X2 245, and I am recording to a non-OS drive, which is a WD 2TB Green Drive. Again, I never had such issues with the 9600. Right when I installed the card, I had a more severe stuttering problem due to conflicting audio device drivers, but that seems to be fixed, yet a minor stuttering still exists. Should I return the card, or is this a software issue?
This sounds like a driver issue. I'd recommend completely uninstalling the drivers and trying again with the latest (maybe just a bad install). If the problem persists, then I'd try some legacy versions. I've used AMD cards dating back to an old Radeon 9200 for watching live TV on my PC and never experienced the issues you're describing.