Cannot connect to Internet, but can browse LAN

We are small shop with about 100 computers, SBS 2003 domain and Cisco ASA 5510 for firewall. DHCP managed by Cisco and DNS by SBS.

We started having random cases where a computer would get IP address, can connect to all LAN resources, but cannot get any external connectivity. Sometimes assigning static IP help, but now this fails as well.

Help!