[NiteWave]

this issue is not a known issue, so I don't reply you at once.

Could this be a setting or a bug with LSAPI PHP Suexec in deamon mode ?

this looks a nice clue and I actually did simple tests in daemon mode, tried to reproduce 503 error --- but failed.

not sure if it's ok for you to run short time tests on your production server.
e.g., switch to non-daemon mode, restart, see if 503 still happen. or disable xcache temporarily and test if no 503.

another good way to strace lsphp5 process, to catch the 503 when it happens.

in debug's point of view, it's good you can easily reproduce 503 error(by graceful restart) in current environment.

also, please give a screen shot of lsphp5 settings, see if we can give any advice, which may avoid or reduce 503 error.