This has been happening for a while now and it's really beginning to irritate me.
Basically what happens is I will open a window, aka arsenal, mail, character, AH, etc... The game will work normally and then randomly the game will hang, normally when I mouse-over something and a tooltip should pop up. The only way to fix this is to open another window.
So for example, I go to the AH and open that window. I try to type something in to search, but it hangs. I press I to open my inventory, and that fixes the problem. I then go to post an item in the AH, with both the AH window and my inventory window open. It hangs. So I press f5 to open my recipes window, and it fixes the problem, but now my screen is cluttered with windows. I close the recipe window, and it's ok, then randomly it will hang again, etc...
Have you gone though any of the common troubleshooting tasks from here? Also, are you a Steam user?