This may or may not apply to you, but I had the same problem just a couple of weeks ago. Image on my Sony NTSC monitor was just fine but the captured clips in FCP HD 5.0.1 were very dark. After a lot of trouble trying to cure it I discovered that my RAID was not recording correctly (files were not writing to the drive and turned up corrupted). After talking to ATTO I decided to reformat and restripe. Problem went away after that. Go figure.