Because the help says for memory limit, don't overtune - and I've got a low enough memory setting in the php.ini. For the process limit, no matter what I set it to it complains that it is set too low in the log files, so I set it to 0, but given that LSAPI_MAX_CHILDREN is set to 3, it never starts more than 3. I've verified this with benchmarking tools hammering the site, so I don't think this is related, but I'm not sure.
The problem with the perpetual 503 errors is that I can't figure out when it happens and it's not reproducible. It just happens randomly and then until lsws restart, but at the same time there is no heavy memory usage, and all other sites are still working just fine on the same server, even with PHP.