Are your System Pref settings set correctly to how your outputting? Meaning if your using Y, RY, BY, you have to select that in the System Prefs, BMD control panel, output.
Secondly, make sure you have one of the BM codecs selected in FCP under the view menu, video playback. Also make sure your external video is set to all frames.
Thirdly, I found out that the Intensity Pro card should be installed in the 4th or top card slot on the back of your machine.
Hope this helps.
OS 10.5.5, Mac Pro 2 x 3 ghz quad-core intel xenon, 9 gb ram, with BM Intensity Pro card