What format are you playing out in from the blackmagic card? I have just the SDI output card and when using the blackmagic presets (which you have to use to get video out of the card outputs) it would drop frames and stop all the time, even with a i7-950, 12 GB Ram, Quadro FX 4800.
If you turn off the GPU acceleration you can then render your timeline and then it will play but that defeats the purpose of using the high end GPU card so you don't have to wait around for renders.
I think Blackmagic has failed to build cards and drivers/presets that work correctly with CS5. If you talk to their tech support they are frankly clueless. They pimp their crap all over and have great prices but their hardware just doesn't perform. I think this issue of dropped frames will continue until BlackMagic gets their heads out of the sand and creates a solution. If you load the Pemiere Pro Benchmark 5 and run that on your system, you will likely be fast enough for most of the stuff your doing, but with it trying to play out through the hardware card, it fails...
You said your video is on Raid. Is that Raid 0, Raid 1???
Raid 0 gives you the most speed but your more at risk to lose your data. The blackmagic disk speed test installs with the drivers, run it and test your drives.. I found mine were plenty fast, but I still had dropped frames with GPU accel, turned on.
You may want to edit in a Non-Blackmagic Timeline taking advantage of the speed of GPU render of effects. Then when you are nearly done, nest that first timeline in a Blackmagic timeline for final playback. You may have to turn off GPU and render the final playback to play out through the hardware.