Am colour correcting in FCP6, after a few minutes of using the excess luma tool the programme crashes. I often get a white or green screen on canvas and external monitor shortly before. This time its XDCAM footage, but I've had it happen with HDV material also.
I had a look through some other threads but found nothing related. Any ideas would be welcome, am getting a bit tired of the auto save vault, besides it reminds me of the garbage compactor in Star Wars (without Princess Leia in it).
The "Excess Luma/Chroma" and the overlays, are an extra burden to FC and the video card on play (this is why they should be off to ensure full quality on play), but I've haven't heard of crashes for this reason.
Mac Pro (mostly 8-cores) have been having lot of troubles to deal with HDV and XDCAM (hundreds of posts the last year), your issue may be related to this. Since the release of SL and FC7 it seems that things have improved.
I won't rcommend you to upgrade (haven't done yet here), but try to optimize your system with DiskWarrior or similar application.