bitflipper
Why does my MixControl say version 3.7?
 
Assuming I haven't accidentally invented a time machine, I'm guessing there are separate versions for USB and Firewire?

Absolutely.

Just came across this and now I'm really confused. Why would Focusrite recommend connecting my Scarlett 18i20 to a USB 3.0 port when the device is USB 2.0? Anyway it looks like there's some good info included to help troubleshoot some issues.
https://support.focusrite...t-has-an-AMD-processor

Focusrite advised using a USB 3.0 port?  I've been using USB 2.0 ports because when I first got the unit, that's what they advised.  I'll try a USB 3.0 to see if that helps.

Do you happen to leave Mix Control open all the time?  After much experimentation, I discovered with my 18i20 and Mix Control 1.8 (and corresponding driver/firmware) that leaving Mix Control open all the time causes similar stability problems as you described.  Basically, the 18i20 seems to lock up and stop responding to the PC after some time and needs to be power cycled to get it back up.  The simple solution is that I only open Mix Control as needed to change settings and then close it immediately.  Haven't had the hardware crash on me since.  This isn't a SONAR issue as I've been able to replicate it with other DAWs.  
 
I did try a recent beta version (pre-b12) to see if it would help with the hardware crash but I hated it due to the latency changes.  None of the settings had sufficiently low latency compared to 1.8 and the lowest latency setting produced audible clicks.  They definitely seemed "beta" quality and not ready for prime time. 
 
I haven't bothered with 1.9b12 because of the warning that the 18i20's firmware will be irreversibly updated.  The other beta drivers I tried were so bad that I don't want to risk that.  
 
 

Thanks Dave76 for that advice.  I do sometimes leave Mix Control open, so I'll try closing it unless I need it open for changes. 

aspenleaf
Focusrite advised using a USB 3.0 port?  I've been using USB 2.0 ports because when I first got the unit, that's what they advised.  I'll try a USB 3.0 to see if that helps.

Yes. They advised using a 3.0 port if you are having issues with an AMD processor. I clearly remember reading at one point where they recommended "NOT" using a USB 3.0 port. My 18i20 is connected to USB 2.0 and other than the slight increase in CPU everything else seems to be okay. 

I use beta Focusrite driver on Win8.1x64 and it is way mare stable and reliable, than official release.

Soundwise - are you finding the lack of buffer settings in Mix Control to be an issue?

aspenleaf
Soundwise - are you finding the lack of buffer settings in Mix Control to be an issue?

Not at all. You can set the buffer from a special dialog, which is a lot more convenient.

This is timely for me, as I was just wondering the same thing.  Two days ago, I wrote to Focusrite Support to ask whether I should consider moving up to the 1.92 beta. 
 
With 1.8, I have a 'pop' that occurs at the start of playback, though everything else works fine.  Once in awhile, I do need to power cycle the 18i20, which I leave on all the time.  Same for the MixControl software -  I leave that on all the time, and it is in my Startup folder.
 
I concur wit the comment that their support is excellent and fast. 
 
I went through much the same situation when I upgraded to Windows 10 over a year ago and then discovered their beta driver site.  The beta driver at that time caused pops, clicks and stutters despite everything I tried, so Support gave me those steps to uninstall everything and reinstall 1.8, which I am still using a year later. 
 
I too am very concerned by that warning that the firmware will be updated which cannot be reversed.
 
I will share any answer I receive.