Server Crashing

My server crashed today with just 500 concurrent users. Yest we could take upto 1200 without going down. I don’t what’s causing this, but I think it’s a memory leak. Something is taking up all the RAM. My error logs (in general, and the logs around the time of crash) show two recurring errors:

1. PHP Warning: apc_store(): Unable to allocate memory for pool
2. PHP Warning: include(/home/XYZ/wp-content/cache/config/master.php): failed to open stream: No such file or directory in /home/XYZ/wp-content/plugins/w3-total-cache/lib/W3/Config.php on line 262

Also, the cache directory on my WP is about 20 GB. Is that normal?

http://wordpress.org/plugins/w3-total-cache/