I need to start by saying I'm NOT a Mac user. However, this sounds like a classic case of your computer processor trying to do more than it can.
A couple of things to try:
First, shut down every other bit of software on your computer that you don't need--especially things like web browsers, wifi and (if you have it--I know many Mac users don't) anti virus software.
Second, under Edit/Preferences/Audio Hardware, try increasing the latency setting. This will increase the size of the buffer giving you more protection against "stutter" but with the downside that, when tracking, latency may become a production issue in monitor feeds.
Third, check that all your audio device drivers are up to date and not corrupted somehow.
Finally, some of the really high quality effects are just processor-greedy. You can improve performance by pre-rendering some or all of your effects. To do this, just hit the pre-render button (the little lightning bolt graphic at the bottom of the multitrack effects rack or on the mixer display). You can turn off the pre render at any time if you want to make changes.