Dedicating resources in Windows 7

I'm running Windows 7 Ultimate 64.

CPU = corei7.

I encode HD video. Up until a couple of weeks ago, I was able to encode a full 1080p movie using MeGUI (set to "High" priority within the program) in about 2.5 hours. Using a resource monitor, I could see that all 8 cpu threads were operating at almost 100% for the entire 2.5 hours. Which explained why it was difficult to do other tasks like movie my mouse cursor fluidly while encoding. Not a problem....it was a sacrifice I made to encode in the least amount of time.

Recenlty things changed. I noticed that the projected time for my encodes bumped up to between 10-20 hours. Looking at the resource monitor again, I noticed that cpu usage was varying wildly between 15% and 75%.

I'm puzzled as to the change. I've been encoding for a few months now utilizing all of my processor power. I went through and uninstalled/deactivated whatever I thought were unnecessary background processes. Is there a way to manually configure a specific program to utilize the max from my processor?

Thanks in advance.