Kaylakaze: I think Jade Empire uses the same engine as NWN and NWN also has this problem. What I read about it is that the engine uses something with your CPU's clock to do timing calculations, but if your computer has been running for several days, the number it's looking at is very large and causes calculation errors. There's loader for NWN that fixes this, but I haven't seen anything for JE. Rebooting really is the best way to fix it.
Edit: Actually, I think it's NWN2 that has this issue.
This appears to be true. That's why a clean boot works, with the other fixes applied.
Windows 8 does a hybrid hibernate/shutdown when you tell it to shut down, which doesn't reset the uptime timer. To work around this issue, restart your computer by typing this into CMD (Find it by searching for cmd in the start menu):
shutdown -r -t 0
It will instantly restart your machine by actually shutting it down, rather than hibernating. This resolved the issue for me.