Don't know your system specs - but I'll take a stab as I've had this problem...
1) cut the first 10 seconds to 5 minutes of your video off and try again - if it works, then you need to recapture that bit from your original tapes.
2) If you are mixing and matching video formats in the same time line and forcing them to 'conform to original project settings' then this problem may occur - try converting each clip to .AVI before assembling your project.
3) If you use a seperate harddrive to capture your raw footage to - get all the files off it and reformat it then relaod - try again
4) If you have a 1 harddrive system, do a complete defrag - the search funtion on this board can give you some guidance in this area.
5) Backup, reformat your system drive and reload - try again.
Hope this helps
My System Specs:
Cameras: 2 x Canon XL1s
Computer: Dual Intel Xeon 2.8Ghz processors (hyperthreaded) - Intel SE7505VB2 mother board - 2 Gb PC2100 RAM kingston -Adaptec FireConnect 4300 -2xseagate 10K scsi hdd 150G ea -2xseagate 7200 ide hdd 200G ea - Plextor PX-708A DVD burner - ATi Raedon 9800Pro video with Dual SUN 21\" monitors - Sound Blaster Platinum live!
Software: Windows XP Pro with SP2 - Pyro Pro (adobe\'s premier pro, encore DVD, and audition bundle)