Photoshop :: Why Does CS5 Extended (Mac) Keep Randomly Freezing

I'm running Photoshop CS5 Extended for Mac. I've had it for over a year now and it has worked perfectly fine until now.
 
Mac OS X
Version 10.6.8
Processor: 2.4 GHz Intel Core 2 Duo
Memory: 4 GB 1067 MHz DDR3
 
The issue started a few weeks ago. I can't think of anything in particular that could've spurred it. What happens is that (generally within a few brush strokes, like five to ten) the program freezes. It gives me the little grey spinning wheel telling me that it's thinking, and that never stops. I've left it for hours before, waiting to see if it would ever snap out of it, and nothing. I always have to force quit the application.
 
There's no particular tool that makes this happen, because it's happened on everything from brush strokes to changing contrast to switching tools.
 
It's not like I'm using giant files—some are large and have lots of layers, but others are small blank canvases. It isn't like I'm running too many programs, because sometimes it happens while Photoshop is running by itself (no other programs.) There doesn't seem to be anything in particular that triggers it.
 
I even tried uninstalling the program and reinstalling it. I had hoped that if it were some sort of bug due to settings I had changed or something, that would get rid of it. It didn't, but I'm inclined to think I didn't manage to remove everything related to the program, because when it opened after the reinstall, it still had all the same brush settings it had before I had uninstalled it. Since I make a portion of my living off of illustration and graphic design commissions, it's incredibly important that Photoshop is usable, and fast.