This is clean install php , no ioncube , no any opcode cache nothing.
Error 503 still exist .
I noticed that the domain which have fewer than 10 characters work well and which are the domain of more than 10 characters long show 503 error
I'd also suggest setting up apache 2.2 mpm event + php5-fastcgi.
Then setup litespeed PHP when php5-fastcgi functions with apache.
Run php-cgi -v & php-cgi -M?/-m you may have it as php5-cgi on debian not sure.
This will show errors too.
Lastly you can strace -p any php processes you see and check the issue. This is how I discovered WordPress w/Missing mods eats up pointless CPU cycles.