Repair your .pst file!
Rather than copy the instructions here, refer to [How To Fix Your Broken PST File](http://www.howtogeek.com/howto/microsoft-office/fix-your-broken-outlook-personal-folders-pst-file/).
It’s not just for use when Outlook stops working – it’s a useful preventative action you should run occasionally to repair little problems before they become big problems. Anytime Outlook starts running slower than normal, give this a try.