Regarding the ctrl+f, if there is a lot of text in the window, mirc might take a long time to search the buffer, making it seem to be locked up. You can adjust the amount of text that mirc will keep in the buffer (hopefully speeding up searches) by going to the options (alt+o) under Other, and changing the Window buffer option. As I recall, that number is how many lines mirc will remember for each window.
This also happens when the cpu doesn't have much spare time to work with, when other programs are especially active. Also, in the past, some programs I've used somehow conflicted with mirc, making it run unusually slowly. For example, opening the mirc options window could take 30 seconds, even though mirc was getting 99% of the cpu time. That also affected my searching. Closing those problem programs returned mirc to normal.
In short, give mirc time; it might not really be frozen, just really busy. If that's the case, try adjusting the amount of text that mirc keeps for the window, and also take note of what other programs are running when it happens.
On the other hand, if mirc doesn't unfreeze after a minute or two, then that's probably a different problem that I haven't encountered.
