I performed a longer test this time, ahout six hours, with mTwitch, irc.chat.twitch.tv, on five busy channels. Unfortunately, there were no crashes during this time. I tested with both a debug version of mIRC v7.46 and the release version.
If you can try again with v7.46, however this time please use the "/debug on" command which will enable writing incoming/outgoing server messages to a debug.log file. Make sure to uncheck "Lock log files" in the IRC/Logging section. The next time mIRC crashes, the debug.log file should contain the last line it received from the server that might be the cause of the crash.