I've got Throat, Choir, Punch, and several other Antares plugins. Every one of them causes Sonar to stutter. Doesn't matter if it's 2 tracks or 20. I sent an inquiry to Antares several months back and never got a response. I can render the mix just fine. The stereo track faithfully employs the plugin with no chop whatsoever.
 
EDIT: I followed the tech recommendations for dropouts, pops, etc. Increased the HD buffer to 2048. (I don't have any active resource hogs running in the background, nor any auto-update stuff.) It eliminated most of the interference but the specs on my rig should preclude any such problems. Maybe I'll whine some more to Antares.

I got this email from Antares, so I'll post it if anyone else has this issue:
 
Unfortunately, there is a problem with the AVOX VST3 plug-ins (including Harmony Engine EVO) in Sonar X3 which we are currently looking into.  The good news is that you are able to use the VST2 version instead. In order to load the VST 2 version into Sonar, please follow these steps:
 
In Sonar go to: Utilities - Cakewalk Plug-in Manager.
- In the Cakewalk Plug-in Manager, under "VST Configuration" click "Options". 
- In the "Options" window please point the Plug-in Manager using "Add..." button to the following location(s):
 
Sonar X3 64bit - C:\Program Files\Common Files\VST2
Sonar X3 32bit - C:\Program Files (x86)\Common Files\VST2
 
- Make sure that the "Re-scan failed plug-ins" and Re-scan existing plug-ins" are checked.
- Press "OK"
- Back in the Cakewalk Plug-in Manager, under "VST Configuration" click "Scan VST Plug-ins".
- Your Antares plug-ins should now show under "VST2 Audio Effects (VST2)."
- You may have to go in have to click on each plug-in individually and click "Plug-in Properties" and make sure that the "Enable as plug-in" box is checked.
 
You may also wish to remove the VST3 version of your AVOX Plug-in from your PC manually.  Here is how:
You will find these individual plug-in files in the following locations on your PC:
 
C:\Program Files\Common Files\VST3
C:\Program Files (x86)\Common Files\VST3
 
**DO NOTE** Make absolutely sure that you don’t remove the wrong files for plug-ins that you don’t wish to remove.  Your computer will be much happier this way.
 
Let me know if you have any further questions,
Henrik Antares Audio Technologies

Thanks for posting! Ran into the same problem and switching to VST2-versions - as the suggested solution says - solved the dropouts.