This problem applies primarily to the first generation Actiontec routers (rev. A-D). The NAT table is significantly larger in the second generation (rev. E) Actiontec, however, it may still not be sufficient for heavy gamers or P2P users supporting many upstream connections. See the following posts:
»MI424WR-GEN2 Rev E Configuration Thread

When a PC behind your router makes a connection to a server on the internet, the router has to "remember" which connection from which PC the request came from, since you have only one external IP address, so that when the response comes back, it is routed to the same connection on the same PC. This information is kept in the "Network Address Translation" (NAT) table.

When the NAT table is full, the router can't remember any more connections and so appears to hang. The two primary culprits in filling the NAT table are torrents, and refreshing large server lists in gaming servers such as Steam.

Firmware 4.0.16.1.56.0.10.7 displayed the results in the Firewall Settings / Security Log with entries like "NAT Error : connection pool is full. No connection created". The recent 4.0.16.1.56.0.10.11.3 firmware release increased the size of the NAT table, but users are still reporting the same problem. The Security Log entries no longer display NAT pool connection errors.

Approximate NAT table sizes