Cubase freezes but playback continues

I’m having this problem that seems to happen completely randomly… I’ll be working in Cubase and all of a sudden the GUI will freeze while the arrangement keeps on recording or playing back or whatever. Sometimes it happens while I’m not even doing anything, like coming back the next morning and Cubase has frozen. Nothing short of killing the task will get me out of it. It seems completely random. Are there known plugins that cause this or an issue with using around 30GB of my ram? (I’m on Windows 7, 128gb ram - Cubase 5.5)