for 503 error, it usually means php crash.
please check lsphp5 setting in
Memory Soft Limit (bytes)
Memory Hard Limit (bytes)
since 1G xcache will be calculated as part of lsphp5 process, so this soft/hard limit should > 1G. if memory assigned to lsphp5 exceed these limit(especially hard limit?), lsphp5 process may be killed by OS(or litespeed process?), so 503 error.