OK I have SONAR Professional on Windows 10.
 
I'm now installing the latest update so maybe there will be a miracle and everything will be great.
 
But in the meantime,  I have:
 
Q6600 2.4 GHz Core 2 Quad (yes it's old) + 4 GB RAM.
Sonar installed on SSD.  Other drives are SATA 3 GB/sec, no IDE drives attached.
I have a Focusrite Scarlett 18i8 USB audio/MIDI interface.
I'm using a Roger Linn Adrenalinn as a drum machine and guitar processor.
 
I set up a 16 channel audio project.
Set MIDI sync and clock to Focusrite, Adrenalinn to clock to incoming MIDI.
Start recording.  I can tell that MIDI clock is glitching and speeding up and slowing down.
It is not possible to follow the drum machine as tempo is varying.
On playback, there are chunks missing of recorded audio every few seconds.  I haven't figured out if it's really missing from the recording or if playback is just really glitchy.
I set the audio IF's latency to 10 msec and did everything else recommended such as setting CPU for background processing, disabling anti virus, turning off Ethernet, etc.  It seems to make it a LITTLE better but still unusable.
 
Now I know Q6600 is pretty old but looking at this comparison to a 3.5 GHz i5:
 

 
The new one is only 86% "faster".  Well, if you took the timing problems of my current system and cut them in half, they would still be bad.  I don't want to spend hundreds to update the system if that's not really what the problem is.
 
And suggestions for how to dig deeper into what's really going on?

Yes, I agree that upgrading CPU wouldn't help. I've upgraded from q6600 recently, it helped in areas where it should help, but not in stability areas. It seems that focusrite drivers were slightly more stable on old system, but can't say it for sure. I have updated drivers too, and it seems that MixControl 1.9b10 works a bit better, but again I'm not sure.
I can see that you have both midi and audio problems. I suppose that it is different problems, and should be solved separately. Midi syncing could be a bit tricky as I know, so try to set up it some different way, or find some other way to check what is wrong.
With missing audio chunks, it should be tested if you can see them without using Adrenalinn, with some other material.

Sorry if this is too trivial: Is it possible that Adrenalinn is not set to sync to external clock, and/or it is sending out clock of its own?

Your clock should be set to audio:
https://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=Synchronizing.03.html
 
Also disable allow arm changes during record until next update:
https://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=Recording.15.html
 
and...
uninstall mix control/reboot/reinstall latest version (or your latest beta if you want)/reboot/reset the settings and the hardware of the interface.
http://us.focusrite.com/downloads?product=Scarlett+18i6
 
In windows control panel -> sounds, disable the scarlett, use your internal interface.
 
Check for motherboard chipset driver updates (intel driver update website?) and update your BIOS. Keep it plugged in a USB2 socket (not 3).
 
Do NOT use ASIO4ALL.
 
Cheers..

Updated previous post to be more comprehensive.

JoseC.
Sorry if this is too trivial: Is it possible that Adrenalinn is not set to sync to external clock, and/or it is sending out clock of its own?

Hey I'll try anything.  Adrenalinn is definitely set to track MIDI tempo etc. as it does start/stop and generally follow SONAR's tempo.  I disconnected MIDI out back to the DAW and it didn't change anything (however miraculously this morning the problem seems to have gone away).  
 
Thanks!

Thanks for all the tips.  I'll keep an eye on it.  The computer rebooted overnight to take some updates, I reset the audio IF, and it seems a lot better now.  Aggravating!  But I'll take better, than what was going on yesterday...
 
Thx,
 
DL
 

Digital Larry
CPU for background processing

You should change that back to Best Performance of Applications. Nothing SONAR does or uses runs as a service; Cakewalk's own CTO, NoelBorthwick has debunked this urban legend. My own experience with it was like yours.
 
If problem continue or recur, I suggest you start by verifying you get predictable playback and recording with internal MIDI and soft synth audio with the Adrenalinn offline.
 
Also, you should clarify exactly what "Set MIDI sync and clock to Focusrite" means in terms of settings. Clock should be "Audio", and Playback and Record Timing Masters should be set to Focusrite drivers. I'm not in front of my DAW right now, so I'd have to double-check MIDI sync settings for slaving an external device, but I'd start by getting SONAR to play back and record correctly by itself.
 
The old CPU should not be a problem. Mine is comparable, and runs great.