Question: I'm working on 3 separate projects in pt9, not identical but similar in content. 2 of them work fine but 1 of them crashes after a second of playing back. This seems to happen anywhere in the edit (if I click on playback elsewhere for example).

Anyone encountered this? quite hard to find the fault through google.

Thanks

Oh forgot to add that I open up an autosave backup and it works... until I save the project again and it fails. :(

Answer: 
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.