Is there anything I can do about this, short of upgrading to Windows 8 (I hear the load is spread more evenly)?

I'm getting to the point in this project where the first core is hitting the red sometimes, causing clicks and stutters. I know I can freeze some tracks, but I'm at a stage in the arrangement where I'd rather not for the time being. 

I'm interested in knowing the answer/solution to your question as well, Sharke. I often see this happening to me, and then I wind up having to freeze tracks in order to avoid that first core from peaking out... the others barely break a sweat most of the time. Seems to me a big waste of resources to start clicking and stuttering when we have power to spare in the other cores.

How about toggling E to turn off all effects and see if it helps ?

I've got this note copied somewhere in the forum for multi core processor setting :

Thread balancing for multi-core processore
To edit inside Sonar->Preference->Configuration file

To edit ini file direct :
\program files\cakewalk\--sonar--version--\cakewalk user data\--sonar--version\
File : AUD.INI -> ThreadSchedulingModel=<0,1,2>
0 = Same as previous versions of SONAR. 
1 = (default) Better thread balance. Model is more efficient and can provide cycles for other tasks. 
2 = Additional worker thread is created. This may result in improvement with Quad processor systems or higher. Not recommended for Dual processor systems. 

Setting ThreadSchedulingModel to "2" seems to have improved matters slightly, but not greatly. It definitely helped. What seemed to help even more, was clearing out some unused synths from the browser. I'd totally forgotten they were there. Not sure how much processing power/memory they take up when sitting idle, but I'm not peaking in the red now thankfully 

some programs see better results if you open task manager and setting priority to high or realtime.

Setting ThreadSchedulingModel to "2" seems to have improved matters slightly, but not greatly. It definitely helped. What seemed to help even more, was clearing out some unused synths from the browser. I'd totally forgotten they were there. Not sure how much processing power/memory they take up when sitting idle, but I'm not peaking in the red now thankfully 

i remember seeing in another thread i think about aufiosnap and rendering esp bounce to clip that the izotope audio algorithms that cake licenses  only use one core. which is why mixing down some audio takes a while.

Maybe it's also partly because the OS is using that first core as well?

I have a problem since changing that thread parameter to 2. Ever since then, whenever I load up the project I was working on when I changed it, Sonar tells me that one of my synths is connected to a silent output and therefore won't be heard. It's not though, it's output to the master bus and sounds as normal. I'm a bit worried (and irritated) by this error message every time I load the project though.