Try another player like Mplayer .
This is due probably to a corruption that must have occured during the download process: missing or corrupted data that prompt your current player to stop reading the video data at the point of the corruption but to keep reading the (uncorrupted or whole) audio data.

__________________
iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-