Why don't you get a PC? I never understood why Macs have half less power then PCs but are twice the price of PCs...
lol. Yeah, buy a PC and ALL your problems will go
Anyway, do you get the system overload message at the beginning (i.e. after loading a project and pressing play)? Or is it all the time during a session?
I have a mac pro 8 core, and I get this damn message sometimes on first startup of some projects, but not all. No problems after I click ok and the audio has been buffered, but it does piss me off when it pops up.
I am more carefull with what I load now, as I used to slam any amount of effects/plugins into the project and just leave them sitting and not removing old plugins/ freezing tracks, and deleting old audio take files.
Post a screen of a project you have issues with, someone may spot something.
Just wanted to let you know that buying a mac pro is not going to rid you of these errors.