You cannot share too many mp3s with MacOS due to the poor performance of Apple's MRJ. Even on a PC sharing thousands of files might cause performance problems because of all the metadata that will gathered from the mp3s (although Sun's Java Runtime Environments will handle a couple of thousand files much better. I can share up to ~5000 mp3s without having freezes on a Thunderbird 800, 256M RAM but the CPU-load will go up to 50% if I try so).

The only solution I can think of is to increase the RAM allocated for LimeWire to a high value like 64MB or even 80MB. If that won't help you can't share that many mp3s.