Cisco Routing Question

Quick cisco routing question, not really security related, but still giving me loads of trouble.
heres the task, configure nat on one of our routers. Easy right? no problem? Well not exactly. You see when we configure nat and specify a serial 0/0 ip address to the ISP(Frame relay point-to-point) the latency is high. So it was sugessted to use the fast ethernet interface with the ip address of the ISP, and use the inside addreses as a secondary ip and than specifiy the ip unnumbered fastethernet0/0 command on the serial interface sub interface. The actual serial interface has no ip address.

When inside the router you can ping everything and anywhere you want. However once you get on the private subnet, you can only ping the fast ethernet public interface(ISP) and can't go any further.

Any ideas?

Which routing protocol do you use ?

Maybe the piece of the routing protocol as well as the ip route part of your running-config would be helpfull here.
While your at it, check the "sh ip route" . Also your nat rules, i'd like to see.

Cheers