We had a similar problem back in November on whatever version was current before the December release. Turns out we had one user with a corrupt inbox. Whenever he checked his email in Outlook the server would go nuts and crash. We lived like that for a week, restarting the server once or twice a day.
We solved it by moving everything to the sole VM inside new, stonger machine, upgraded to the December release, and had the user delete their account from Outlook then re-add it. Things were smooth until the most recent upgrade. We're adding more RAM tonight.
Mik MullerMontague WebWorks