I have confirmed I get the 503 even when APC is disabled for the particular site.
Is it possible that when a user hits the CloudLinux max CPU limit, LiteSpeed throws a 503 error? I know the way it is supposed to work is that the site should only slow down, and CloudLinux support confirmed that an error would never occur, but I am not totally convinced. When I check LVE history the accounts do reach the max CPU allowance around the same time the 503 error occurs.
mistwang: can you confirm that maxing out the CPU limit will not cause 503 errors from LiteSpeed?