have you enabled php suExec ?
this is a known issue for apc/xcache with php suExec.
so in shared hosting environment which php suExec is enabled, only recommend eAccelerator, since it stores opcache on disk, not in memory.
There are some detail discussion regarding this issue on this forum, you can search it out.
you can modify apc.php code, to print current process's pid and its parent process's pid. you will see current process's pid keeps changing all the way -- this will help to understand its behavior completely.