H.264 encoding question--dissolve playback hiccups

I am doing 2-pass VBR encodes that seem to work well on all recent computers. However, on an older machine, the dissolves between a series of stills causes audio dropouts and hiccups in playback. What do I need to adjust to fix that? Lower the peak VBR rate? I can't quite figure out what settings fix these problem spots that only appear with a slower machine.