I'm seeing the same issue on my FreeNAS box, running the latest client 1.3.77 (x64) for FreeBSD. It doesn't appear the the client is actually crashing; rather, after ~10 minutes of heavy data transfer its CPU usage (as indicated by 'top') goes to 99-100% for its main thread. After that point, the client web address can no longer be reached until the jail or service is restarted. Would be glad to provide further information to help debug as the client is currently unusable.