When using VPN across the AT&T platform it is often necessary to lower the MTU (Maximum Transmittable Unit) setting of the VPN client or if using a concentrator then the egress port to the AT&T U-verse RG to 1472. Common symptoms of MTU needing to be lowered would be VPN connections not being stable or having very poor performance for applications and browsing that traverses the VPN.
Often lowering the MTU on your end alone will correct this; sometimes it may additionally require adjustment on the far end as well. Also, a note on the Motorola NVG510-VPN connections using PPPTP or L2TP will require setting up the GRE ALG option (or PPTP etc.) in the NAT/Gaming section.