How long is the project? Also, please mention the format and resolution of the bulk of the clips: HD, 2K, 4K? If 4K, try exporting at HD or 2K resolution.
Problems can often be tracked down if a project is shared. It's easy, takes but a few minutes and can be private.
Here's how... http://nch.invisionzone.com/topic/23659-tips-for-getting-help-on-this-forum/
If it contains sensitive material link it in a personal message (PM). Click the envelope above. It won't be passed on or retained.
Use FILE|SAVE PORTABLE PROJECT AS, not simply SAVE PROJECT, and don't forget to share it.