1.check if mysql is the bottleneck when WaitQ much greater than 0
2.if mysql ok, then the problem is php can't keep up with the big number of incoming requests / second
opcode cache will help
try run lsphp5 in daemon mode
3.if still can't keep with, set up another physical machine, clone current server's settings, but share a database. i.e., if the traffic still increase, have to consider the cluster solution.
the above is from openX experience.