And the little blue circle keeps spinning.

Good morning. I've just about finished a song with more tracks and plugins than I've ever used. MIDI and audio Aux tracks with ezdrummer 2. I'm real pleased because it sounding good! But, once in a while the screen turns opaque white and the irritating little blue spinning disk shows up. (No offense intended spinning disk!). There is also a popup saying "Pro Tools is not responding. Close program"! What are some possible causes of this? I have just gone through and verified the Windows 7 optimizations and all is good there. When this happens I just click on "close program" and restart the session so nothing is lost but It's aggravating! I'm on Pro Tools 9.0.6, Win7, 32 bit. Thanks