I'm now getting tons of scratching (hesitating) and audio dropouts on the same setup (PC, interface, etc.) that used to be fine.  The only thing that I can think of is that I have added more stuff so that there are more background processes (there are a lot on the resource monitor).  I killed the antivirus and as many processes as I felt I safely could, still same problem.  Any hints?  Any way of figuring out which processes are creating the problem?  Thanks!

Check whether this also happens with other audio/video programs like You Tube, Windows Media player...I wouldn't be surprised if this is a general problem as I have it myself. It is probably DPC Latency (Deferred  Procedure Call)
If so then it is not related to Sonar but has to do with drivers like ndis.sys, tcpip.sys and scsiport.sys. Disabling WIFI or Ethernet card (i.e. Realtek PCIe GBE Family Controller) will often help.
If you have a multi boot system make sure that in The BIOS only the relevant boot disks are enabled and not your data drives.
Make sure that you do not have set any power saving mode, set the PC to high performance.
Check the power management of USB Hubs, it must not be allowed to turn off the device.
Do the same for your network card.
Search the internet for DPC Latency to find all kinds of solutions. You will see that especially gamers have big problems with it.
On the internet tools are available Like Latency Mon (
http://www.softpedia.com/get/Others/Miscellaneous/LatencyMon.shtml
) and DPC Latency Checker (
http://www.thesycon.de/eng/latency_check.shtml
)
https://www.sweetwater.com/sweetcare/articles/solving-dpc-latency-issues/
https://www.sweetwater.com/sweetcare/articles/pc-optimization-guide-for-windows-10/
https://www.sysnative.com/forums/windows-7-windows-vista-tutorials/5721-how-diagnose-fix-high-dpc-latency-issues-wpa-windows-vista-7-8-a.html
 
Regards
Jan

In addition to Jan's excellent general advice for Windows, in SONAR check under Preferences  > Audio > Sync and Caching and experiment with the buffer sizes. Also when the dropout Toast notification pops up, click on help and go through each of the steps to test for possible causes. 

Can you add anymore ram? I just did 2 days ago and it solved the very same problems that you mention-nice. Good luck!