The preacher wore a shirt with grey, black and white vertical stripes, and I've got all kinds of chromatic abberations / 'prism'ing' driving me NUTS! Is there a way to get rid of this?

I mean, short of turning it to b&w?

i'm running FCP 5.1 with stock filters/plugins

The offending rainbow effect is called moiré. Do a search of these forums. There is a discussion going on elsewhere that may help you.

Try a gausian blur filter. Start with the lowest value possible and creep it slowly upward until the artifacts disappear.

You sure this problem isn't caused my a missmatch in video formats? Are you mixing different video formats? If so, what are they? Or have you shrunk the footage in the timeline? (check to see if it's been scaled)