One thing to keep in mind. There are times when a program can freeze because another program is not responding. I've had that happen, where a program such as Firefox stops working and another program (or programs) freezes. Looking in task manager, Firefox shows normal (0-2% CPU usage) while the other program shows 50+% usage. Killing the program task for the program with 50+% usage just moved that usage to another unrelated program. However, killing Firefox caused everything to start working again. That may not be what you're experiencing and my example has happened with programs besides just Firefox, but it's something to consider. Try closing all other programs and see if you still have the problem. ***And only test with plain mIRC and NOT with UPP loaded/running***
Beyond that, give any details you can about what else is happening with other software at that time (such as virus scans, automatic email download in a program like Outlook, etc), what softare is running, number of networks and channels are open (and total # of people in those channels), your system specs (cpu, memory, graphics card, etc), how often it happens - try to get specific times on it... is it every X minutes exactly, or does it vary, for example. Basically, just give as much information as you possibly can so people can try and figure out what the cause is.