Could you try typing this into your chat window and reply with the results please
//echo -a $os $version $iif($beta,$beta,0) $md5($mircexe,2) $file($mircexe).sig $alias(0) $script(0) $dll(0) $com(0)
Other info that would be great to know is which server are you experiencing this on, is it specific to that server or is it global? Does it happen only on one channel or on several/all? Does it always happen or does it happen occasionally? If occasionally, are there any steps you've noticed to make that happen?
If the problem persists, try launching a new clean version of mIRC and see if you can make it happen again.