Mac Pro :: Applications Freezing / Stalling And Forcing Reboots

For the past month or so, I've started to have some really weird problems with my Mac Pro. Applications will suddenly stall out or freeze up (endless rainbow wheel), and Force Quit won't work to quit them. It happens most often with iTunes and Finder, but it's also happened with Firefox once or twice. The whole system doesn't freeze or stall - I can generally still use another application (i.e. if iTunes stalls and won't quit, I can still continue to use Firefox, etc.), but generally if I leave it this way too long another application with eventually freeze, forcing me to shut down by holding down the power button (Shut Down won't work in these cases).

Occasionally when this happens, I have left my computer alone, and sometimes it unfreezes or unstalls by itself, but this is usually hours later, and sometimes it doesn't do this even after hours of waiting. I ran a hardware test thinking that faulty RAM might be the issue, but the hardware test said everything was OK. I don't know if I should believe that or not. I've tried repairing permissions and verifying all disks, internal and external, and everything checked out OK. What is going on here? Why do things keep freezing/stalling and forcing reboots?

Here is what I've got:
2x2.8GHz Quad-Core Intel Xeon
2GB 800MHz DDR2 FB-DIMM
Running Mac OS X 10.5.8
I have really normal stuff installed - MS Office, Photoshop, Ableton, etc. No really weird third-party applications other than some really simple stuff. I'm not the type to overwhelm my computer with a bunch of crap.