Are you pasting input? More specifically are you pasting unicode input? mIRC usually freezes the first time it tries to render unicode if your font does not support unicode and font linking is on (might still occur without font linking as well). Note the default font (fixedsys) does not support unicode.
Also, in mIRC type:
//echo -a $md5($mircexe,2) $script(0) $dll(0) $version $os
Report the results here.
If you're running any scripts you should reinstall a fresh copy of mirc without scripts and verify that it still happens.