Make sure you have the system NAT address defined properly in processor IP connections, and in the set you have NAT set. If you do not have the public IP defined correctly in the 5000 it could also be why TFTP doesn't work quite right... This is not in System->IP Settings, but in System->Devices and Feature Codes->IP Connections->P6000 (typically, the p6000 part might vary slightly) then set NAT IP Address to your public IP address.