Rendering - cores stop working

[iHartVideo]

This is a strange problem, I'm wondering if anyone here has expereinced something like it. I rendered a short 4 minute video twice yesterday, both times I encountered the same issue. I'm rendering as a 3mbps WMV file.

I begin the render, things are going fine all 8 cores are going pretty much full blast (I have a CPU usage widget).
Then, about half way thru it gradually drops to about 1 core working at 30-50%. By this time I have nothing else running, and I'm not even using my machine... just watching the rendering progress & the CPU usage. The first half (approx) of the video renders in a couple minutes, the last half takes about an hour.

Any ideas?

Computer stats:
i7 920
12 GB ram, 7GB dedicated to Vegas
64 bit Win 7
64 bit vegas - 9e

 

[aituv]

What are you rendering? What type of file format and what are your project properties/settings?

 

[iHartVideo]

I tried again after rebooting and it seems to be working correctly.

Strange.