I'm not much of a synth user, but I'm working on a project where the team wants a particular synth sound and so I am exploring what's available to me in Sonar X3. Rapture and Pentagon are both more complexity then I want to get into, so I'm checking out DropZone and Z3TA+ 2.1
DropZone is real transparent and easy to use, but the filter and LFO sections are just a bit too limiting. 
I really like the flexibility and ease-of-use with Z3TA+, but find that when I put it in a project, it just gets unstable...Audio Engine stops and occasional complete crashes. At one point, the project I was working on stopped playing ANY synths after a Z3TA crash and I had to drop back to a previous day's backup...lost a few hours of work...and never did figure out why/how the MIDI info-->synth path got broken. I have done a couple of tests with brand new projects and always get an audio dropout within 10-15 minutes of the time I insert Z3TA+, and it seems to deteriorate. I can make it happen by a couple of multi-clicks on virtually any parameter...this causes no problem in any other facet of the Cakewalk SW. 
 
I have set it up as a standalone and practiced using it live for quite some time, with a couple different very CPU-intensive programs, and it performed flawlessly. 
 
Other than this, I have had nothing but rock-solid performance with software synths using X3e and my current relatively high performance system (see below). 
 
Any thoughts on what might be the issue??
 

Go into Sonar preferences. In the MIDI section there's a setting for the size of the MIDI (not audio) buffer. If it's set to 250 (the current default) try increasing it to the old default of 500 or even more. Some systems seem to need it set over 1000.

The buffer tells Sonar how far ahead to read MIDI tracks so it has time to get everything sent to the right place and in sync. If the buffer's too low then dropped notes and all kinds of MIDI madness can follow.

This may or may not fix your specific problem but increasing that buffer isn't a bad idea in any case.