PHP / MYSQL connection failures via socket under moderate load
Error 11 in unix is a segmentation fault error. Either the server has bad memory chips installed or you are making other configuration values too much for the server to handle.

I once experimented with setting the nofile value in linux to a value like yours and my whole server crashed. Now I make nofile set to maybe 40000 tops. Try that, then reboot the server to see if the problem goes away.