Okay, I'll try a different port, disabling the XP firewall, etc. per that sticky post.
By the way, when I connect to the same server and room using Chatzilla I never get discoed/parted/whatever, so whatever the issue is, it's specific to mIRC. And the problem still exists after selecting those reconnect options as per my last post (although at least now it does automatically rejoin the channel after a second or two).
*scratches head*
