I would guess the most likely reason for your performance issue, given the spec of your Mac is pretty good, is media drive speed.
How is your raid connected ? Firewire 800, for example, is only really good for a single stream of HD - after that it will possibly start to choke. (Use the AJA drive speed tool to check your drive speed)
So if you regularly do jobs where there are 2 or 3 or more shots on the screen at once, or multicamera stuff, you might want to look to using a faster interface.
Additionally when FCP starts doing odd things a good idea is to trash the preferences and re-set it up. (FCP pref manager does this quite well - free download)
That clears out some buggy behaviour that often cant really be explained, and speeds things up a little.
Finally which ProRes codec do you use ? - obviously 442 HQ and 4444 demand high bandwidth and again will give playback issues unless you have very fast drives.
Obviously some, or none, of that may apply in your case and you may well have tried most of it, but hope that helps.
best wishes
Neil
http://www.patience.tv