VPN up Browsing down.

I have a T1 line with a covad netopia router and a multitech rf830 vpn router. The rf830 acts as the dhcp host on the peer to peer network. Very frequently all computers that go through the rf830 lose internet browsing capabilities but are still able to ping outside sources and connect to each other and the VPN connections to our 4 remote locations stays active. The only way to restore browsing is to restart the VPN router. I have connected directly to the covad t1 router during a time when the browsing went down to test the net and was able to get a solid connection. I have swapped out the rf830 and have upgraded the firmware and still the problem persists. If anyone has any advice pls help. thanks