I've had this problem for many years now in all mirc versions (starting from 6.16, i didnt use mirc before that version) combined with all scripts i've been running. The biggest script used is the famous nonamescript. The other scripts are smaller ones which are also used by some other people. My box is a bare windows 2003 box certified spyware free and virus free and without antivirus or other low level software running. There also arent any weird drivers installed. The box runs two instances of mirc. Both experience this problem though one instance is experiencing it a lot worse than the other one. Its impossible for me to keep mirc running for more than 24 hours or something. After that period one of the mirc instances usually crashes (Not Responding) because the non-paged pool memory threshold gets crossed and all network connectivity is lost. The network is restored instantly when I kill both mirc instances at the console or even one (as long as the non paged pool counter gets below the threshold again). Sometimes the instances dont crash yet and are still able to report an error like: "Unable to connect to server (No buffer space available)". After the non-paged pool threshold is passed there are lots of errors in the eventlog with Event ID 2019 "The server was unable to allocate from the system nonpaged pool because the pool was empty.".
I've supplied a screenshot of my Task Manager in front of a (temporarily-because-of-high-cpu) non responding mirc window on that box so you can check out the non paged memory leak counter and also the insane high cpu load this one mirc instance is always giving me at certain times (see CPU and NP Pool column at the Task Manager window in the screenshot). Both mirc instances have the same scripts installed and nothing differs except the irc servers they connect to and the channels that have been joined. The mirc instance which is experiencing the problem a lot worse gets disconnected often of the servers it wants to connect to ((re)connecting is automated in some script) and its my belief these disconnects happen because of the almost 100% cpu load at those times. I would give my right arm to get this finally sorted cause I would very much like to connect to this windows 2003 box remotely without having to restart mirc each and every day but I must admit I dont have much faith left this will ever be solved.

Screenshot link: http://img256.imageshack.us/img256/7862/cpunpua3.jpg
Specs:
Windows 2003 Service Pack 2 with all updates
AMD Athlon XP 2000+
1.5GB RAM
256MB ATi Radeon 9200

If I can be of any assistance at all by doing some tests or providing more imformation by all means contact me or reply in this thread.