What on earth is going on?

I think you need to give more details about your project setup because I use Kontakt all the time and do not have the issue. 

All recording has been fragile - we have moved from tapes which wear out, tape heads which wear out or get magnetized and need cleaning, tapes that stretch and have wow and flutter TO
Computers where operating systems can be prone to virus and other problems, hard drives with their problems, software issues, etc.
 
Having said all this, there is not enough info in the OP to indicate that this has anything to do with Sonar.

+1 to Bundlefly's comment.
 
The reason why any program can appear "fragile" is because there need to be preferences that can accommodate a wide variety of systems. It's necessary to match the preferences to the system. You may get lucky and find the factory defaults match what you have, which leads to the comments like "Program X works perfectly in my system but Program Y" doesn't, while in a different forum someone is posting that "Program X works perfectly in my system but Program Y" doesn't."
 
This isn't just a computer thing. Anyone who's lived through biasing a tape recorder for different tape formulations has experienced a variation on the same phenomenon. 

 
I searched " The play *bug* with sonar" and this is what google returned (first result)
http://forum.cakewalk.com/Solved-MIDI-playback-bug-Sonar-Platinum-201605-m3433639.aspx
 
More often than not it is buffers. but possibly its a hardware issue.
I would run some tests by disconnecting the MOTU and use onboard audio (just for some tests)
Try the WASAPI driver for Windows 10. If the crashes stop you know its hardware related.
I would start from gear 1 and work my way through the whole system. there's something causing those crashes.
I wish I could tell you its all sonar, I just don't think it is. Very stable here and im not just saying that or trying to rub it in the wound im just trying to help

dan le

eph221
Half the time, samples that were working just stop sounding.  This happens in Play and Kontakt.  I'm getting really sick of this.  Sonar is just so fragile.

Hey I think your issue is legitimate.
I  Sonar, sometimes the midi channel is changed, and I don't know why.
If suddenly it does play, can you click on the midi track (right side), and do Alt 8 to see which midi channel the track is on.
Perhaps, it has been changed, so instead of 1 it is now something else other than 1.
If it is 4 then on Kontakt, click on Midi Ch and change the number to the corresponding channel of the midi track.
I found this out after so many times that Kontakt will not play, just like you described.
Try it and let me know if it works.
dan

Hey Dan,thanks.  Unfortunately, that's not the problem.  The other annoying thing is that if I start playback somewhere other than the beginning, some samples play, other's don't  It's a crap shoot everytime I play something back.