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.
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.