It could be connected to a plugin. Next time it happens, make a note of which VSTs and VSTi you're using at the time. See if you can narrow it down to one which is in use on every crash.

Another thing which PT doesn't always seem to like is multiple versions of the same plugin running. For example - if you need two reverbs, try to use two different VSTs instead of the same one loaded into two different slots.

ROG.