Reinstallling LW for mac means you can install over the top of the previous version or just drag the previous LW folder to the trash. The incomplete files' preferences are stored in files called download.dat & bak & are found in the incomplete folder. So just leave your LW preferences as are, leave the LW downld folder & incomplete folder as are & you can re-install.

How much ram do you have by the way? And how much spare HDD space do you have? I wonder if that's related to your problem.

If you have same problems after re-installing, then try deleting LW's Preferences folder (whilst LW is closed) found here: /Users/your_username/Library/Preferences/LimeWire