I did a recent shoot on the water front and beaches around Florida. I used a lot of overcranking 60 fps, 720P, playback at 30P.
On my computer monitor, I noticed the blue's of the water and greens of the trees fluctuating, like there was a color switch going on and off between these colors. I haven't finished outputting this project to a final video, so maybe it's just a preview problem.
Filming beach scenes, boats, seagulls, and kite surfers. Very Bright sunlight and reflections on the water. The images look great other than that. It doesn't happen on my normal 1080P 30FPS recording/playback.
It seems to me there are a lot of preview-related issues with XDCAM footage. For instance, I was doing a slow tilt across a lake at a power plant in a beautiful clear blue sky, no wind. When playing my timeline in Final Cut Pro, it looked like the colors were shifting in a way that reminded me of some people's "rolling shutter" issues. Final output, however, exhibited none of the problem I described. Thank God too, because it was a pretty nice looking shot!
Keep in mind that FCP is doing real time processing to display the video in your Viewer and Canvas unless you them set to 100%. FCP is also displaying only 1 Field (note worthy if you're shooting interlace).
I don't see the issues you speak of though with Mac Pro 8 Core, 8GB RAM, ATI Radeon 2600XT GPU though.
You really should be looking at output on broadcast monitor (or at least HDTV) rather than judging what you see in FCP.