Jim,
The issue may not be with your outlook program but your mailbox size. Try creating a backup of .pst file, by going through control panel --> mail. If you no longer have access to that feature, I start to quickly run out of ideas. You may also try the method mentioned here: http://www.slipstick.com/problems/repair2gbpst.htm

The repair function while lengthy should not revert you back to a clean version of outlook so that might be an alternative although annoying one.

Ever try thunderbird?