Can't play 1080p videos

[ekisha]

I have tried different methods (filters, renders, software) but still can't play 1080p mkv videos. 720p plays just fine but 1080p stutters & the whole video plays in slow motion. The only player which could handle 1080 mkv was Totalmedia theater but it didn't let me change the audio and pick embedded subtitle (both were deactive despite the file having them). I don't know how to fix it. Any ideas?

Here are my specs:
Intel Core i7 960
MSI x58 Pro-e
ATI Radeon 6800
6 GB Ram

 

[PhilFrisbie]

Make sure you have the latest video driver installed. Also try VLC for playback.