Doubt it's a dns issue. If you're using plain IP addresses and not hostnames, DNS servers are never queried so they can't be an issue (unless of course a master server is involved that needs to be communicated with for whatever reason, then there might be an issue).
If the fake players are using tcp, it could be windows limiting the connections. To verify this, goto control panel > administrative tools > computer management > event viewer.. and check the system and application (cant remember which one) for error 4226. It will say something about a max limit being reached for 1/2 open connections.
If you are seeing this error, you will need to patch your tcp.sys file. The most known application for doing so can be found here --
http://www.lvllord.de/?lang=en&url=downloads . I get this error when I run scripts that connect to a lot of servers via tcp, and I have to patch tcp.sys everytime i install windows. It can also happen if you have a lot of active torrents, or do a lot of file sharing. But most games use UDP so this might not be the issue.
However, if they are Linksys routers, they could be what is blocking the connections without telling you. I had a linksys router do this randomly (like it was detecting a flood), and I never found an option to turn it off, so I ended up using a pc with m0n0wall as my router which doesn't do "hidden things", and has worked perfectly for years now. I wouldn't doubt that many other SOHO routers block things like this too, as a security feature, and dont give you the option to turn it off.
You may also want to disable any software firewalls (windows firewall for example) that could be blocking the connections.