Hi,
Based on your situation, please follow the below steps to check the result:
1. Try to start Outlook in safe mode to check if the issue still occurs. (click run and type outlook.exe /safe )
If the issue would be gone in safe mode, maybe you have an add-in that is causing trouble. Please disable the add-in (File->Options-> Add-ins).
2. Check if there is something wrong with the Mouse settings(Control Panel > Mouse) or your mouse? Try to restore the Mouse settings and try another mouse, then check the result.
Refer to the following KB to get more details:
http://support.microsoft.com/kb/266738/en-us
Hope this helps.
Best Regards