Under Preferences > MIDI > Playback and Recording, under Playback increase the MIDI Buffer Size. The default is too low for most systems, and this fix has been referenced numerous times in this forum as solving a bunch of baffling MIDI issues. Try 750 ms or even 1000 ms. This doesn't cause delays or latency issues.

Anderton, I should have mentioned that I already tried changing the MIDI buffer size. I tried 500 ms, 750, 1000, and 4000 and it made no difference. I also tried doing Sonar resets on MIDI and the audio engine. If MIDI buffer size was the culprit, I'd expect to hear similar dropouts with other soft synths, and I do not. 

Roger on the buffer size. This is embarrassing, but I always use the VST2 version because that's what I've been using for years...didn't realize there was a VST3 version until I read your post. The VST2 version has always worked fine for me, so try the VST2 version and see what happens (and I'll try the VST3 one and see if I experience the same issues). Also, you say you are evaluating it...is there a current demo version? I assume you have the latest SONAR demo that incorporates some of the VST3 handling improvements. but the z3tz+ 2 has also had several updates recently.
 
BTW many people have reported issues with Arturia soft synths running with VST3 and recommend using the VST2 version (which is what I use with them). FWIW I've had no problems with Steinberg, Waves, or iZotope synths running under VST3.

I installed Cakewalk's Command Center to download and install 30 day trial versions of Sonar Pro and the Z3TA+ 2 soft synth. It looks like Command Center is their perferred/enforced method for doing so. I don't think it gave me an option for installing VST2 vs VST3. The plug-in DLL is named Z3TA+ 2_64.vst3, so it's a 64-bit vst3 plugin for sure. I don't see a way to download the vst2 version of it on the cakewalk website.