Make sure the song is WAV
Make sure video is Windows Format
Make sure your PC has NO background tasks, and has MAXIMUM Virtual Memory output.
Make sure your PC has a lot of RAM.
WMM can do wonders when you learn how to fully use it... trust me, I've seen what it can truly do myself. It can be professional, but has its bugs. And as they said, save as a small vid every once in a while and check out how you do. When it freezes, though, when you find the points in the timeline in which it freezes, pause before it, and through it. It works for me.
LIKE, 404, D00D! Signature not findaficated!