Well, I "solved" the problem by switching to another IRC client for awhile. Now I'm a channel op and need to use mIRC for some scripts that I need to run. I have a different computer, and did a fresh installation of mIRC 6.3 on it.
Everything was fine for about three weeks then the issue started again: I'd get disconnected from all channels I was in, but not from the server. Auto-reconnect to the channels worked fine though.
Last night I worked with an IRC friend who's been an mIRC user for several years. It turned out to be an issue with my router/WAP/NAT firewall combo box (Netgear MR814 v3) that only affected mIRC! Weird.
So as a workaround I've bypassed the router and just plugged my DSL modem's crossover cable directly into my PC's LAN jack. So, the moral of the story is that when you're having weird connect or disconnect isues with any program, try bypassing/disabling all firewalls as a troubleshooting step.
