I’ve had similar issues. I put it down to using Skype; but that could be just because thats about the only thing that might be taxing my wireless. I posted about my issues here: http://forum.mikrotik.com/t/rb751u-2hnd-crashing-when-using-skype/53367/1
The next step is to try a netinstall and resetup and see if it still happens…very frustrating!