It would be interesting to see what support has say about it. I see no indication support was called. They might take the call now even though the product is out of support.

OK, einstein, I'll try that as one last shot....
Will let you know what happens....

Can you share a copy of the project file somewhere? We still don't know what synths are involved and that's an important factor.

I'll be damned - an Uninstaller for Command Center, but no Uninstaller for Sonar Platinum!!!!
Sorry, guys, a DAW should NOT be this hard or difficult to problem solve or even manage.
And with that, I flush all Sonars down the toilet....

jtmusicAnalog
I'm ready to say - Exit Sonar, Enter Logic Pro. I wanted a Cakewalk product to work with my brother, but, I can figure out a way (with various export options) we can meet in the middle and work together (without Sonar on my PC.)

Of course do whatever works for you, but if what you were experiencing was due to SONAR, this forum would be loaded with angry people who experienced the same thing. Because they don't, there's something unique about your situation. Therefore, it's an issue that can be fixed. Not sure how much time you want to devote to fixing it, but calling support has been the solution for many people. I'll also agree that 1) migrating settings for something with problems is probably not a good idea, and 2) there's a good reason why TASCAM now creates their drivers in-house...

The uninstaller for Platinum is in the Command Center. Anything installed by the Command Center is maintained by the Command Center.

jtmusicAnalog
I'll be damned - an Uninstaller for Command Center, but no Uninstaller for Sonar Platinum!!!!
Sorry, guys, a DAW should NOT be this hard or difficult to problem solve or even manage.
And with that, I flush all Sonars down the toilet....

If you're still around, locate & rename your aud.ini file, let sonar build a new one the next time you boot it up.

Alright alright alright.....
For starters, my last tests, I had disconnected the Tascam unit, and did not turn on the Oxygen controller. So, logically, those 2 items were out of the picture, and I still got the stuck notes when stopping playback. May I assume those 2 items are not the culprits?? I vote yes.... Additionally, it was pretty random as to which particular VSTi was "stuck", so that leads me to conclude it isn't a particular VSTi's fault. Can we agree on that?
@ Mr. Anderton: Fortunately, I do have time available to figure this out, but do I have the patience for a DAW that craps out straight out of the box and takes lots of effort and help and posts to resolve, whereas other DAWs are generally good-to-go from the get-go. On the other hand, I try not to be a quitter, and if you guys say this is fixable, I'm still willing to give her a shot. But my patience is a little on the thin side here.
So, I am first going to install Pro Audio 8, see how that flies, then maybe Sonar X3e.
I did "clean" my system of all remnants to Cakewalk... (including the Aud.ini file mentioned in a post.)
I'll keep you posted...

jtmusicAnalog
Alright alright alright.....
For starters, my last tests, I had disconnected the Tascam unit, and did not turn on the Oxygen controller. So, logically, those 2 items were out of the picture, and I still got the stuck notes when stopping playback. May I assume those 2 items are not the culprits??

 
Hard to say, the drivers are still installed. For example with ASIO4ALL, just having it installed on some systems, even if it's disabled, causes all kinds of problems if drivers for a particular audio interface are also installed. You can see if the drivers are visible in Device Manager and disable them temporarily.
 

Additionally, it was pretty random as to which particular VSTi was "stuck", so that leads me to conclude it isn't a particular VSTi's fault. Can we agree on that?

 
Yes, that's definitely a meaningful test 
 

@ Mr. Anderton: Fortunately, I do have time available to figure this out, but do I have the patience for a DAW that craps out straight out of the box and takes lots of effort and help and posts to resolve, whereas other DAWs are generally good-to-go from the get-go. On the other hand, I try not to be a quitter, and if you guys say this is fixable, I'm still willing to give her a shot. But my patience is a little on the thin side here.

 
Totally understood, and I admire the fact that you're being persistent with this. I still wonder if the best option isn't to try the Platinum demo with the clean system rather than trying other versions. Then, if a problem occurs, call support. I believe they can do a remote desktop thing and find out what's happening. Many forum posts that start with "intractable problem, tearing my hair out, SONAR sucks" end with "called support, it's working now, thanks."
 
When I had a pervasive problem years ago that I simply could not fix despite hours of trying, Obedia did a remote desktop thing and the problem was solved once and for all in under five minutes. I felt stupid for spending so much time on something when people who really know their stuff were ready and available. 
 
Also FYI, there is an uninstall option for Platinum. In the command center, right-click on SONAR Platinum and select "uninstall."
 
Remember, support has seen just about everything. It may be this has happened many times before and a fix is already in their database.

I had lots of problems with midi when I had ASIO4ALL installed, even when I wasn't using it. If that is anywhere on your system at all, get rid of it before doing anything else!