Movie problem

I seem to have a strange problem when watching movies. Sometimes the picture somehow speeds up a bit as if it’s trying to catch up with the audio. It happens for about less than a second and it’s ok for a while then i happens again.
Any idea why this is happening? Is it possible that some filters from the Nimo Codec Pack give me the trouble or is it the player because Crystal Player doesn’t seem to do that. Unfortunately the PRO version has annoying message and the FREE doesn’t have the advanced features?
BSPlayer and WMP9 have the problem.

More information? Like the media format (.avi, .mpg etc.), where you got the file (downloaded, created), what codec it is created with (both audio and video), if you have changed it in any way since obtaining it (converted, cut, merged, and with what programs).

The files are .avi, well the problem exists for any avi. It doesn’t matter which codec was used. No haven’t changed it in any way, just copy it over LAN and play it. As if the stream isn’t fas t enough, if i can explain it in that way. But how can the hard disk be unable to provide a stream fast enough when nothing else is active except the player and i HAVE NO other software in the tray.
Tried to lower post processing level, no success.

are lots of programs running in the background? maybe the processor is having problems keeping up

//what processor type/ram do you have?