When the maximum amount of entries in the NAT tables are reached (by connecting to 512 different servers in less than the timeout period, such as updating a game browser or heavy p2p use) the Homeportal simply refuses to service any further connection attempts until some entries free up on the NAT table by current connections timing out. This may take 10 minutes or more depending on the type of connections in use.
Under the NAT section on »home/tech/configuration.html there is a setting for TCP Timeout and UDP Timeout, setting these to the lowest possible values (5 minutes, and 1 minute respectively) will help mitigate these issues. However if you try and update a game browser that has more than 512 servers listed, it will refuse to show you any servers beyond that 512th server on the list because the Homeportal is not making connections.
The workaround for this issue is to limit the game browser to using only 7 packets per second (7 pps x 60 seconds = 420 packets, which leaves almost entries in the NAT table free to prevent overloading)
One of the game browsers I know will allow you to limit the number of packets per second is the All Seeing Eye: »www.udpsoft.com/eye2/index.html It supports a whole lot of games, and allows you to limit the packets per second to 6 or 7, this, combined with setting the UDP timeout on the homeportal »home/tech/configuration.html to the lowest possible allows an effective work-around. This will allow you to see every server on the list beyond 512 servers, however it's going to be a lot slower than letting the list update the normal way.
Note: In firmware version 3.7.1 and above, the NAT table was increased to 1024 entries to help reduce the table overload problem.