Are you streaming your video? If so, may be a network settings issue. I had an issue like yours (not as bad) that was related my network settings (had Flow Control disabled).
Is there any format you can play?
What are your Audio settings in MC? Try WASAPI Event style. Do you send the audio/video to an AVR or directly to TV/Monitor?
In Tools>Options>Video, do you have hardware acceleration On or Off (if ON and your graphics doesn't support it, it will make a mess of video palyaback).
The video files are located on a local hard disk. Actual file transfer speed is around 80 Mb/sec, this should not be a problem at all...
I cannot play any format properly. I have just installed XBMC and the same video files are working just fine.
Music (flac mostly) is playing fine on the MC, it's just the video that keeps breaking up, stalling, sound is clipping and so on...
I am using WASAPI because I have an Antelope Zodiac+ DAC connected via USB.
I tried WASAPI Event Style with no luck either.
I also tried with hardware acceleration on and off, same problem.
Since other software does not have any problems with the same files, I believe it is an MC problem...
Is there a error log file or something for debugging?