The video file cannot be played (on Galaxy S)
Android's built in decoders are rather picky about the files they will play - limited encoded resolution (possibly less than that of the device's display), limited bitrate, limited formats.

If these are new files, try encoding them to be less demanding.

If these are files that used to work, see if something on the phone is perhaps slowing it down - on on rare occasion I had a file that generally plays start failing part way through.  A reboot might be worth trying.