Try removing ALL plugins*, by going to Macintosh HD > Library>Application Support> Digidesign. There, move all items inside "Plug-ins" to "Plug-ins Unused." If ProTools works better, quit and move a few plugins back to "Plugins." Try ProTools and see if it works. Repeat this process until Protools fails. One or more of the most recently added plugins may be the culprit.

If you find one plugin that is causing this problem, download the latest version from the developers website and/or reinstall.

* Note to ProTools HD users, the plugin "stereo mixer" must remain in the original plugins folder for ProTools HD to launch.