again and again and again check your appropriate tcp values
http://fasterdata.es.net/TCP-tuning/ip-sysctl-2.6.txt
and eventually raise them
(tcp_max_syn_backlog and somaxconn)

but you can also have some firewall that put limits

and again and again, with more that say 300 connections, you should
think about changing your software layout, move to n tiers with
connection pooling, it will use less ressources, your system will be
more effective without the cost of open close all that connections