Yeah, so, when I try to watch a x264 (also called h264) encoded HD file (its a .mkv file), I get good audio, but the video is useless. (the video is a 720P HD capture). It looks all "blocky" and "choppy" (and generally unwatchable) like you're watching satellite TV during a hailstorm. How can I fix that?

heres what mplayer complains about:
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Error while decoding frame!21 ct: 0.013 9/ 9 ??% ??% ??,?% 4 0
[h264 @ 0x876b98c]concealing 2720 DC, 2720 AC, 2720 MV errors6% 5 0
Error while decoding frame!72 ct: 0.018 17/ 17 128% 8% 4.4% 6 0
Error while decoding frame!33 ct: 0.024 20/ 20 119% 7% 4.1% 8 0
Error while decoding frame!12 ct: 0.022 21/ 21 113% 7% 4.1% 8 0
[h264 @ 0x876b98c]concealing 2720 DC, 2720 AC, 2720 MV errors6% 8 0
Error while decoding frame!41 ct: 0.016 29/ 29 106% 8% 3.6% 9 0

heres what vlc has to say about the situation:
===============================================
VLC media player 0.8.4a Janus
[00000293] a52 decoder: A/52 channels:6 samplerate:48000 bitrate:384000
No accelerated IMDCT transform found
[00000301] ffmpeg decoder error: more than 5 seconds of late video -> dropping frame (computer too slow ?)
[00000301] ffmpeg decoder error: more than 5 seconds of late video -> dropping frame (computer too slow ?)

Any ideas?

(BTW, I can play a 1080i HDTV captures just fine [.ts files]. They look fan-freakin-tastic.)

I had a problem playing back mkv files with vlc. I'm using Ubunto distro. I found that if you change the access filters in vlc dump from 32mb to a way higher value (i did 512mb) the slow video playback issue resolves and the high resolution mkv files i play 1080p/720p have no more problems. Sometimes you need to play around with the default preferences to get the best result!

BTW if you think you have any problems with your vlc and if it is compiled correctly just visit the www.videolan.org(I'm in no way connected to them!) site. They have linux support for alot of distro's