Make sure you can reproduce this with all drivers up to date.
Then please provide some informations like:
-How is it happening? You're not doing anything, mirc is the active window and just resize itself?
-Can you reproduce this with another window/program?
-Could it be a script doing it? Type "/remote off" anywhere in mIRC to disable all your scripts, stop all your timers (by typing "/timer* off" anywhere too) and see if you can still reproduce.
Note that mIRC isn't set up to resize itself randomly, as far as I know, and that you are likely the problem here (you as in, not mIRC).
