Hi, was pointed to this thread as I have the same issue. I noticed early on that mirc would crash only when there were Japanese characters on screen. And only when the chat window was active. It doesn't always happen, but when it does, Japanese characters are on the active chat window. At first I assumed it was font or otherwise utf8 related. But as others have reported problems specifically with win7 and system locale set to Japanese, I'm now leaning towards that theory as well.
I'm running Windows 7 Ultimate 64bit and a clean official version of mIRC v6.35 (technically not anymore as I now use gtsdll, but I only got that recently and mirc was crashing prior to that). I don't download files with mirc (or at least never have when mirc has crashed) and I'm not using a SSL connection. Before Win7, I was using 32bit Win XP with system locale set to Japanese and this sort of crash never happened.