(If you're a PC QA/repair type, ignore this post as you already know what you need to be doing.)

Couple things you can do to see what's going on with the system in general.
  • Download CoreTemp (just google it). Run that and watch your CPU core temps and make sure they're under 65C under max load. They actually should run around 30-50 depending on how good your heatsink is.
  • Grab a temperature monitoring utility for your graphics card. They're all different depending on the manufacturer and model so just grab one that'll show you.
  • Open up Task Manager (right-click the task bar--> Task manager), go to the Processes tab. Make sure you have "Show processes from all users" checked. Go to the "View" menu and select "Select Columns..." Check the boxes in there for "GDI Objects" and "Handle Count." Then watch those two columns while playing. You shouldn't have anything eating up more than a thousand GDI or Handle counts except for your web browser, your multiboxing app, SVCHOST.EXE (and only one svchost should have that many open) and things like a web server app. If you see something with a bunch (like tens of thousands or more) then you've got a software app that's leaking.
  • If you don't see anything obvious with those, then you've got to look at graphics bugs or BSODs (aka bugchecks).
  • For graphics bugs, ATI and NVidia make stress test apps you can run to exercise the graphics subsystems. If the system hangs when running those then it's likely not WoW or software and is probably a graphics driver or hardware problem.
  • For system crashes, it gets more complicated. Is your system rebooting on you or just straight shutdown until you push the power button to turn it back on? If it shuts off and stays off, that's 99% guaranteed to be a heat problem. If it just reboots, it _might_ be heat but it is probably something else, like a RAM issue or something more gnarly.
Something else to do now before you start messing with it.
  • Right-click "My Computer" and select "Properties"
  • Select the "Advanced" tab.
  • In the "Startup and Recovery" section, click Settings.
  • UNCHECK the box for "Automatically restart" and change the "Write debugging information" from "none" or "Small" to "Kernel memory dump."
That'll prevent it from automagically rebooting and will let you look at the STOP code when/if it bluescreens on you.

See if any of that helps and then get back to us when you find out more.

Cheers,
Ugh