There is a most likely a corruption in your PST file.
1. Close Outlook
2. Go to /program files/Microsoft Office/Office 12 and run scan SCANPST.EXE as Administrator. The SCANPST.EXE file will be located in a different directory depending on what version of Outlook you have and whether or not you have a 32-bit or 64-bit machine and if you installed the 32-bit or 64-bit version of Outlook. Most likely everyone has the 32-bit version of Outlook installed. If you have a 32-bit machine the program will be installed in "Program Files" rather than "Program Files (x86)".
Office 2003 (version 11):
C:\Program Files (x86)\Microsoft Office\Office11
Office 2007 (version 12):
C:\Program Files (x86)\Microsoft Office\Office12
Office 2010 (version 14):
C:\Program Files (x86)\Microsoft Office\Office14
3. You will find your PST file (if you have not changed it). The PST files are usually located here unless you have changed them:
(You can check this location by using FILE->Manage Data Files in outlook but don't forget to close it after).
4. Allow the file to scan and fix the issues.
5. Restart Outlook.
This can also be caused by anti-virus scanning IMAP traffic. When this occurs, restart your computer and see if the issue persists.