I'd be surprised if SONAR turned out to be the culprit, since it's not in charge of plugin graphics. These symptoms are usually specific to a particular graphics card (although the fix may have to happen in the plugin rather than the video driver). If you have graphics acceleration turned on, you might try disabling it. If the plugin supports GPU acceleration, try turning that off.
Now, if you don't see the problem in other DAWs
on the same machine, then SONAR's culpability would be an inescapable conclusion. But if you're comparing against Logic, or any other DAW that's installed on different hardware, then it's not an apples-to-apples comparison.