Sonar, as far back as I can remember never gave me major problems like audio engine dropouts and program closing prematurely.
I don't even know where to start.  Tonight I created a new project from my normal blank template.  I had just two plugins open, Superior Drummer 3, and Kontakt with the new Orange Tree Samples' Evolution Rock Standard (guitar). With no efx on any tracks or busses.
It's a new Sonar build, a new version of Superior Drummer, and a new guitar sampler for Kontakt.  And once again, I got the Audio engine dropout, followed by Kontakt freezing, and then Sonar closing.  When it happened, I believe I was going through various presets of the Evolution Rock guitar, and playing them a little at a time.

In my previous post, I was having issues with the render operation (to a stereo wav file) stopping halfway through.  But now the problems seem to be getting worse, with the simplest of projects with just a drum track laid down, and me playing along on the Kontakt player....not even recording anything yet.

I've been having issues with Saffire MixControl software changing ASIO Buffer sizes on it's own.  So I contacted Focusrite and he gave me a beta driver to try and fix that issue.  I have a Saffire Pro 24 audio interface connected via Firewire.
I usually have the ASIO Buffer set at 512.  And my project settings are currently set to 24 bit/48,000  (I used to have it set at 24 bit/44,000)

So I don't know, these Audio Engine dropouts and program crashes could be a dozen different things.  I thought I might have been overloading my DAW.  But there should be no reason for this to be happening with a new project with just two plugins open.
It could be a driver (but from what software/plugin?), it could be a hardware problem, my interface, a hard drive?   But everything else on my Windows 10 64-bit machine (32 GB ram) is running fine.   I'm totally at a loss. 

And it couldn't have come at a worse time, I was just preparing to launch a local advertising campaign for my recording studio/producer work.  I can't have clients coming over to a DAW that keeps crashing. 
And this is the worst kind of problem, it's intermittent and occurs over various configurations.

Where do I start?

This can have so many reason (incl Windows or hardware failing) ... first thing I'd try would be to restore the full system image (not a Sonar rollback) from a month ago when everything worked fine. You have got such a backup, don't you?

This would rule out software that was changed or updated. If the situation persists, you need to check each piece of hardware ...

I had the same thing happen to a lesser degree after the Windows update of a couple days ago. I installed the latest Sonar version, and all was well. I hope the fix is equally simple for you!

Run the free "picky" Resplendence Latency Mon program.  It may point to the problem.
http://www.resplendence.com/latencymon
 

Thanks for the suggestions.  I already have the latest Sonar Platinum release.  In fact I was thinking of rolling back to the previous release to see if that might help.
I'll also try the other suggestions and I'll keep the full system Windows restore as my last option.
But first I'm going to try the Latency Monitor and see what I can find out.

Thanks again.  I'll keep trying different things, but as I said this problem is going to be hard to track down because of it's intermittent nature.

I've been experiencing the issue with Focusrite (Scarlett 6i6) changing buffer sizes, but Sonar has been pretty stable. I have had some problems with NI Komplete Kontrol software crashing while browsing and switching presets from the hardware, but that's something that's happened in standalone mode as well as when loaded as a plugin in Sonar or FL Studio, so I'm pretty sure it's specifically the KK software/hardware that's causing that one.
 
SD 3 hasn't given me any problems as far as I recall.

I'll chime in because you mention Focusrite.
I don't have much experience with Firewire (I've never used it), but you might consider trying USB?
I was having the same problem as Tripecac on this thread where (for me) since the Windows 10 Creators Update, audio was dropping out every 45 seconds or so (in everything, YouTube, MP3s, etc.).
Since the Creators Update Windows 10 seems to have an issue running the 2i4 (which is USB 2.0) through a USB 3.0 connection. All of my external connections on my PC case are USB 3.0, so I had to run the Focusrite 2i4 directly to the USB 2.0 pins on the motherboard and the audio dropout issue in Windows was fixed instantly.
 
Before that, about a year ago, Focusrite also sent me a driver to get my Focusrite to work in Windows 10, just before they updated to their latest USB driver version.
 
When you say new build, I wonder if you have recently upgraded to Windows 10?
Please note that the Windows 10 Fall Creators Update is due in just 2 days on the 17th of this month, so it might be good to do a fresh ISO install of that when it's available, in which case holding off until then could be a good thing.. 

fireberd
Run the free "picky" Resplendence Latency Mon program.  It may point to the problem.
http://www.resplendence.com/latencymon
 

After viewing the link provided, I realized I already downloaded this utility a few days ago.  And ran some tests with it.
Now this may or may NOT have anything to do with my current problem, but on October 8th, I had this result:


But today, running the same utility for 30+ minutes, while running Sonar (several different projects of varying size)
I got these results today:



Does this have anything to do with why I'm getting Audio Engine stoppage AND Sonar shutting down prematurely? I don't know.
Maybe I'm providing too much irrelevant information.  But I wanted to include anything out of the ordinary AND anything that has changed recently.

For today, running Sonar and trying to replicate the same problems as yesterday haven't been successful.  I've had no problems today in recording or playing back music in Sonar.   I'm sorry I can't provide more clues to determine what's going on.  It's very frustrating.

My 2 cents worth post was retrieved from the automated spam and inserted before your post.

GjB
My 2 cents worth post was retrieved from the automated spam and inserted before your post.

I'm not sure what you mean?  I posted this message in the Cakewalk forums.  What kind of spam are you talking about?  Thanks!
EDIT: Whatever spam you're talking about, maybe it's because of the link someone posted to the Latency Mon application?