Unfortunately tracking down issues like this one, which are rarely reported, can be difficult because there are so many points of failure.
In the past, issues similar to this have been caused by anti-virus/security/firewall software, specific brands of router, router configuration, internet providers monitoring your network connections and disconnecting if they suspect them to be due to virus/trojan/bot related or if they suspect the IRC server you are connecting to, and so on.
You would need to eliminate potential causes one by one. For example, if you disable/uninstall your anti-virus/security/firewall, does that resolve the issue? How about restarting your router? Or resetting your router configuration?
It is very odd that this issue would affect only one specific IRC network and none of the others.
