You should also install VLC (don't setup as default). If the problem still exists then it's a software issue somewhere else.
Most likely issue is the video player or linked decoding software.
*There are TWO main way to decode movies:
1) CPU (software) decoding
2) GPU (hardware) decoding
If the GPU is supported for that codec (i.e. H.264/AVC, MPE2 or VC1) and you also have the DXVA setup in your settings in the video player for hardware decoding it will use your graphics card.
You should try toggling this on and off.
You can tell if it's doing hardware decoding by right-clicking-> "filters" (MPC-HC). It will say "MPC Video decoder" (as opposed to something like "ffdshow video" for software decoding on the CPU.)
**Also, CCC for my last AMD card was enabling Advanced Video decoding (such as flesh tone) that really screwed up my video look (grainy etc). I disabled all the Advanced Video in the Catalyst Control Center.
Software decoding doesn't utilized the Advanced Video features.
- try VLC as well
- try toggling hardware acceleration ON and OFF