I'd suggest you add the pix max_conns option for the static statement of the web server.
For an example. Limiting the number of 20 simultanous connections and 40 embryonic use
static (dmz, outside) 1.1.1.1 2.2.2.2 netmask 255.255.255.255 20 40
access-list outside permit tcp any host 1.1.1.1 eq 80
Please see the following
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_sw/v_63/cmdref/s.htm#wp1026694