There are different level of encoding for H.264 files.the higher level are more difficult to decode. Sometimes the files you find out there are encoded at an unnecessarily high level. It doesn't make them look any better. iI just makes it harder to decode. You don't say what your system is, or what your using to play it back so I can't give you more specific advice. I'll say this, download MediaInfo. It'll give you more information. Look after the frame rate under the first video stream. It'l tell you what level it was encoded at. The other thing to try is to repack the MKV with MKVMerge. Some times the MKV is just messed up. If you rebuild it, it will then play properly.