One tip, especially after a bunch of installs, is to simply do a reboot. Sometimes, memory gets a bit hashed up with all that install activity and it may have an effect on audio operation and interaction. So, before throwing in the towel - try a clean boot first.

Tripecac
Hi, thanks guys for your help!  I'm still trying to fix it but at least I have several things to try.
 
Here are some quick responses to your suggestions/questions:
 
1) I installed the "Focusrite USB ASIO Driver 4.36.5" driver from Focusrite, downloaded from this page: https://customer.focusrite.com/downloads?product=Scarlett+2i4 -- Their site says that the Scarlett 2i4 is supported by Windows 10.  When I use the Device Manager to look at the driver properties for "Focusrite USB Audio" it says the Driver Version is 4.36.5.612, so it looks like the Focusrite driver is correctly installed.
 
2) The Focusrite is indeed the only audio device enabled in Windows.  I disabled the motherboard's Realtek audio and NVIDIA audio via the Device Manager.
 
3) Motherboard is ASUS PRIME H370-A and the CPU is an Intel Core i7 8700 (6 core) 3.2 GHz
 
4) I've been using the Scarlett for general audio for a couple weeks now, with no problems.  None of the games, podcasts, CDs, etc. gave me any problems.  The first dropouts I've heard were today, with Sonar.
 
5) My new computer's Plugin Layout in Sonar has lots of disabled/grey/missing items on the new computer, so I am having to manually re-create it.  While I'm at it, I'm installing some other non-Cakewalk, non-Native Instruments plugins I forgot to install earlier, like Addictive Drums, Addictive Keys, etc.  It will take me a while to get my plugins (and plugin layout) downloaded, installed, and configured on the new computer, so any further testing will need to wait until that's done.
 
6) I plan to test the same "problematic" Kontakt bass patch (mentioned earlier) in both Sonar and the standalone Kontakt 5 application.  This will let me get a better sense of whether it's Kontakt or Sonar (or the interaction of both) which is involved in the dropouts.
 
7) Question: Are there Kontakt settings that I should look at, in terms of reducing the chance of dropouts?  Or are all performance-related optimisations handled by Sonar and the ASIO driver?
 
8) I have changed the Power Plan to "High Performance", but have not re-tested yet (since my plugins are still re-installing). 
 
9) After I test "High Performance" I will try the "Optimise for Background Services" option.  I've done that on other computers before, but hadn't done it on this computer yet.
 
10) My old MIDI USB device (Edirol UM3-EX) does not work with Windows 10, so I've been using the MIDI ports on the Scarlett.  This is one change from the old setup.  Do you think by using the Scarlett as both a MIDI and Audio device is asking too much?  I can't imagine that MIDI is very demanding, though, compared to audio!

I think its pretty clear that you are doing all the right things. I don't think you will have any more troubles. 
You also picked the right processor.  I have the same one in mine. 

I know this is going to sound weird, but I'm convinced sometimes software needs to learn it's environment. So things that crash/malfunction a lot in the beginning start to work fine after a while. Especially when there are many 3rd party apps involved.  I'm sure that there are reasons for this that are beyond my understanding, like buffer memory getting set up. That and a couple of reboots help also.
 
My point is, I've experienced what happened here many times, where things just started working.   And yes Kontact has it's own settings (buffer) you might want to adjust.
 
 

Regardless of whether or not the problem comes back, it's always best with any computer to go into:
 
Control Panel / Administrative Tools / Services
 
Stop anything from "Automatic" start that doesn't need to be. Usually change it to "Manual" rather than "Disable". For instance, anything having to do with your scanner/printer, anything having to do with iPod and so on.
 
Control Panel / Administrative Tools / System Configuration
 
Uncheck anything unnecessary such as anything made by Adobe, iTunes, Quicktime, Java Updater, and again printer / scanner subprograms. This won't stop any of those things from working, but it will stop their subprograms from running constantly. They aren't necessary and will only hurt performance. This latest version of iTunes can be especially bad when it comes to disrupting wave recording.
 
Also look under the "Services" tab in Sys Config and uncheck anything there that's similar as the above.
 
And of course disable system sounds. They will usually run at a different resolution that what you're recording with and this causes conflicts. And needless to say, get rid of all the fancier stuff on your desktop display. I wrote this for Windows 7, but I imagine most of it holds for 10 as well.
 
https://yahoobuckaroo.blogspot.com/2011/03/adjusting-desktop-display-for-best.html
 
Bill

https://www.asus.com/ca-en/Motherboards/PRIME-H370-A/HelpDesk_BIOS/
Version 0904 2018/09/29
https://www.asus.com/ca-en/Motherboards/PRIME-H370-A/HelpDesk_Download/
Version 10.1.17695.8086 2018/09/266.85 MBytes
Chipset
Chipset_V10.1.17695.8086_20180702_WHQL
 
The above are key to a good running system !
 

Inside Kontakt you will want to go to the Engine tab and set the CPU overload protection (by killing voices).  I use "relaxed" but you might want to try the other options.  If Kontakt is killing voices it will make pops and clicks.  You can also change the number of voices an instrument uses max on the instrument itself.  For lots of commercial products the settings are too low and will cause lots of pops and clicks (Project SAM instruments, I'm talking about you!).  For Multiprocessor support set it to as many cores as you have.