music from VSE not synced with soundtracker animation?

i have a music file that i used soundtracker on, and it generated a very nice ipo. i put the music in the VSE, and turned on “Multiplex Audio” etc. no problems. but when i render (or even in the 3d viewport) the music is synced with the animation for the first little while, then s-l-o-w-l-y slips out of sync, until the end of the song where things are totally out of whack. any suggestions?