If the file is very big but short in time it could be also the issue, use handbrake for encoding :
handbrake.fr/
use those setting :
* Container format: MP4, headers at the beginning of the file (for seeking)
* Video format: H.264, Baseline profile, 480x270 pixels, around 400/600 kbps (kilobits per second)
* Audio format: AAC, Low Complexity profile, 44.1 kHz stereo, 96 kbps
you can read also this :
www.longtailvideo.com/support/blog/13898...oding-best-practices