What version? What's the resolution of the bulk of the source files? What are your PC's specs: CPU, GPU, RAM, free space on C:\?
Try this VP version. For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info.
THE best way to quickly track this down is for you to share it. Takes little time and can be private. Have your second project fully on the timeline. FILE | SAVE PORTABLE PROJECT AS per the instructions here. Don't upload the exported video.
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 get a shareable, public link - not one that is restricted.
Someone will check it out.