Aside from Shaun's good advice, you may also have a corrupted fade or file (I've seen all sorts). If clearing Prefs etc. doesn't work try these:

First, try deleting the Fade Files for the session... PT will rebuild them. Corrupted Fade Files can cause all sorts of weirdness.

Second, try a "Save Session Copy In" with all audio files. If there's a weird filename (which can happen and wreak all sorts of havoc) it will tell you and rename the file properly.

Lastly, create a new session. Import tracks from the old session one at a time. Play your timeline. If it crashes right after importing a specific track, then something in that track is off, and it will be easier to find -either media or a specific plug-in could be the issue.

when in doubt: trash all preferences, repair disk permissions and restart.

another trick you can try is to make all tracks "inactive" (note that this is not a "voice" selection idea, you do this by right-clicking a track or through the track menu), play back for 10-15 seconds, then reactivate the tracks. 9 can encounter a memory caching problem, and this will sometimes clear it up.

What version are you on? I started to have a lot of crashes too when i updated to 9.05 Previous versions worked much better.

also tries to delete the folders "digidesign database"