With Win7-64, the first thing that I would look at would adding RAM. That 4GB is just about enough to run Win7-64, and not much more.
Next, do you have only one physical HDD? If so, I would look into adding at least one more, and place your Projects, media and Scratch Disks onto it. A third HDD would be a good idea too, and then separate your media, from your Project and Scratch Disks.
What are the specs. of your Source Files? If AVCHD, or any other flavor of H.264, then the i5 just might never you really smooth playback.
Did the Mercury Playback setting accidentally/randomly get dissabled?
Actually, As we said above we have been working on this project for 5 months with no issues. The i5 and the computer in general have been amazing.
The only thing I can think of that changed in our work flow just before it stopped responding while playing unrendered files is we had to throw an iphone video on the timeline.
Before I could play unrendered timeline files; now I have to render it for smooth playback.
I checked the Mercury setting and it is greyed out.