Welllllll….Outlook is a resource hog. You don’t give details as to processor, RAM, HD, etc. Have you investigated CPU usage, RAM installed, tweaking virtual memory, etc.
When a reboot solve this problem, the first thing I would look at is diminished resources. This can be due what you have running simoultaneously, or just not enough RAM on the system.
Has this surfaced recently with no changes to the box? Has this been an ongoing thing that has recently worsened?
More info please.