I don't really know if this applies to mac, but I had the same problem using Windows. I solved the problem by making Windows allow my internet connection more simultainious connections and now the problem is gone. 10 was default there and it didn't really help until I set it at 40+ ending up at 65

Try only allowing 1 or 2 downloads at a time in Limewire, and then try browsing to see if thats your problem as well