Thunderbird hangs

Recently, my well-used and up-to-date (31.0) Thunderbird has been experiencing long hang-ups during many normal operations. Deleting messages, moving messages between folders, sending messages, and once, retrieving a message. It hangs for 30 seconds to several minutes, displaying the "busy" cursor. Windows reports "Not responding." Task Manager can't kill it until it decides to come back. Actually, it can kill the ThunderbirdPortable.exe process, but not the thunderbird.exe process.

This is not specific to TB Portable, as many other TB users have reported similar problems, but all the proposed solutions have been ineffective. I'm posting here in hopes that this community may have some ideas.

It's so bad that I'm going to have to move to a new email client if it isn't resolved.

Update:
TB seems back to normal, at least for the moment. I switched virus checkers and found a problem disguised as a windows installer. TB then ran but reported unable to copy to certain local folders. I found 3 folders which had no msf files, and the msf files were not automatically rebuilt when restarting TB. However, doing a "Repair Folder" from each of the files' Properties dialog successfully rebuilt the msf files, and all seems back to normal.