This might sound really simple, and I'm sure you have checked it out, but your hard drive isn't nearly full, is it?
You might also be running out of ram if you have too many items in your undo list. Either delete undo hisytory, or maybe try having less undo steps available. 8 gig of ram is not much use to Logic unless you are running 64bit.
I can't say I have ever had this problem. My home setup is a 2007 2.2ghz Intel core duo, 4 gig ram, and it does OK with all but the hugest projects with lots of VI's. Logic 9.1.6. It just works.
Not much help. Sorry.
If you are having CPU issues, you need to examine which plugins are causing it. Some Instrument plugs can be extremely CPU -intensive.
My projects load up and play through fine for a short time, then the cpu monitor spikes and won't go down until i shutdown logic and sometimes the mac itself...It's so bad on this machine that the whole mac slows down for a good five minutes even after i have closed Logic.
This sounds like there could be some more fundamental issue with your Mac and it's OS. You way be able to track down some of the issues by having a look in Activity Monitor to see which elements are using (and continuing to use) CPU power.