Maybe more info would be helpful

1.) Style, Key and BPM of song
2.) specific realtracks instrument (number, name) giving you the problem.

If you recreate it, is it OK? That wavering sound could be caused because another process in the computer (like anti-virus software) is grabbing too much of the CPU while RB is playing.

Kevin

Now at bandcamp: Crows Say Vee-Eh @ bandcamp or soundcloud: Kevin @ soundcloud