Network Contnuosly loses connectivity with firewall in Place

I have a network of 30 hp DC dx2300 PC’s IP add DHCP assigned; 1xhp DL380 5150 server with SBS2003 (Domain Controller, DNS &DHCP); 1xhp DL380 5120 server running an application that runs on SQL2005 & MS 2003 Server. All the Computers periodically lose connectivity to the servers until a repair on the network connection or restart. They do not all lose connection at the same time. I have realised that that if i switch off the firewall and router, the network works fine. i have removed the firewall, only router in place but no network problem. Firewall is CISCO PX 501.