Going to need a lot more info before anyone can really advice. Full specs on the computer, the exact OS, the exact version of Pro Tools are the very least. Posting a Sandra Lite report is a good idea(read the link above called
Help Us Help You for details on that).
After that, how big is the session? What is the Playback buffer set to? What plugins are in use(especially on the Master track, if you have one yet)
