s2s VPN Disconnection

I am not very good in VPN and of late one of my vpn connection keeps dropping. The VPN originates from a Cisco 2801 Router and terminates into a Cisco ASA 5510 firewall. Initially the VPN was terminating into a Cisco 506 PIX but when we migrated to the ASA, it keeps dropping. When we reboot ASA, it comes up.Can anyone help me with what to check on both ends?