"Pro Tools quit unexpectedly" a lot!

[MidnightFlyer]

I am on Snow Leopard, 2011 iMac 9.0.5

immediately after closing about every fifth session, or so, where it says "file" on the upper tool bar,.that hangs for a few seconds, then I get the message that Pro Tools quit unexpectedly" and I must re-launch.

Luckily this always happens after the session, when I am closing it, so I never lose anything, But this problem is so vague that I can't figure out why it's happening, and it's been doing it for year, and now it's doing that after about closing every third session.

Of course I trash preferences, etc.

I won't re-install at the moment cause I'm in the middle of a project and I've already suffered with iLok all day yesterday.

so where should I look to make that annoying glitch go away? any ideas?