PMP450 VPN Problem

Hello,
when I use 450b client in NAT and PPPoE enabled I’ve a trouble:
if I’ve router with pptp client inside LAN and this make connection, vpn connection ok but after a few time 450b block dns traffic, or a arp table stop. (ping to a IP ok but resolving not!)
Next If I stop vpn, connections back online.

Any idea?
Thank you