It's generally due to OSX's Java which is not very good. (SUN should design it like they do for all other platforms!)

Try these things:
1. Delete LW's preferences folder whilst LW is closed; found here: /Users/your_username/Library/Preferences/LimeWire

2. Tip: Purge the Downloads Queue!

3. Keep your shares less than 2,000 on OSX. OSX's Java has problems dealing with too many incompletes & too many shares.

* How much ram do you have? How much spare space do you have on your OSX drive?