Problem: Linux dom3 servers keep dying.

[Gandalf Parker]

OK now we are just shooting in the dark.

2 out of 4 games?
Have you checked the port numbers you used against your services list? Usually...
cat /etc/services

Or have you run netstat when it happens to see if something is kicking in on the same port?

You might even check a more complete services list such as
https://isc.sans.edu//services.html
to see if your ISP is blocking some port "for your protection". Ive had some games run into problems with that altho usually its the players ISP since Ive taken great pains to have no friendly filtering on my connections.