I have wireless as well, and if mine loses signal nothing happens to mirc at all, i wouldnt be supprised if its something underlying mirc thats giving up the ghost, and either crashing or causing mirc to crash, some wireless component thats not acting right when you loose connection.
I have a linksys wireless net and have not had any such mirc crashes when the same thing has happened (not that its happened often tho, so maybe its just fingers crossed).