Hourly comm. of a CISCO router in VPN

Situation: NT 4 client, ISDN 805 router through VPN to 2000 domain controller, Exchange 2000 server and SMS server.

Router opens a dial-up connection to VPN every hour, sometimes more, due to a ARP reply to its VPN router.

How can I kill the ARP reply’s in the router?
IOS version 12.0(7)