What can I do?

.mkv 720p files are not playing smoothly, it just looks like I would have some old 486 processor...

Even on laptop those files are not moving quite soft with VLC player, when THERE ARE specific MOVING scenes... like walking you can see delays (picture by picture)

Just tell me WHAT will solve this problem? HONESTLY. I just bought WD HD TV media player today and installed latest firmware! VIDEO and picture is amazing, but not happy with the moving...
(this isn't a big thing ...but if you compare watching DIVX and MKV is a BIG DIFFERENCE!, divxs plays so smootly)

Let me know!

If it's not playing correctly on VLC or WDTV, then it's probably a corrupt file. I've played dozens of 720p MKV on my WDTV and never had a problem.

Please post the Mediainfo for the file and then someone can tell you what's happening.