I've run the tests,
I haven't been able to reproduce this with Suexec deamon deactivated and Xcache enabled, or with Suexec deamon activated and Xcache disabled.
To get a 503 error you have to be running a php instance at the right moment.
To reproduce this I run a gracefull restart and as fast as possible reload a web php using PHP multiple times.
Out of 10 tries I managed 5 503 error messages with Suexec deamon + Xcache 3.0.1.
Should I try Xcache 2 and disable varibale cache or is there a way to debug why this happens with Xcache 3.0.1 ?
It rearly makes sense to be able to allow variable cache… and it's stable so long as there isn't a gracefull restart.