You are using the drum machine 'D16 Nepheton'?
Is Sonar loosing sync with this external device. I will presume so, if that's the case.

simpleman
You are using the drum machine 'D16 Nepheton'?
Is Sonar loosing sync with this external device. I will presume so, if that's the case.

I gather you Googled it and got fooled by that mock image of a hardware device.  D16 Group is a software company, and the Nepheton is strictly a plug-in.
 
Getting back to the issue, my considered opinion is... I dunno.   I can't say I've ever heard SONAR users complain of MIDI tracks sent to the same synth going out of sync with each other. Previous reports of looping issues have always been about soft synths going out of sync with audio tracks and the audio metronome or possibly with another soft synth.
 
Again, I would ask whether you can reproduce the issue with another drum synth. If not, I'd suggest you hit up the D-16 forum.
 
 
 
 

Yep, I concur & I tried a few things using different drum VSTs but when I also loop a section, over time my loop timing degrades.  As soon as I restart, it's back in time.
 
I haven't tried Sharke's suggestion though so I'll give that a go & report back.
 
I'd also really like to solve the issue as I would use the feature more if it worked!
 
It's interesting to note that I use 8.5 Pro & you're using X1 or 2 & the problem is still there.

Definitely try the metronome thing. MIDI timing drift when looping is a known issue and someone from Cake told us about this workaround ages ago. I use it and have never experienced the problem since.

Enabling metronome in my demo case has no effect. But I just checked, and changing the project sample rate to 44.1kHz from my usual 48kHz fixes it.
 
I'm curious to know what sample rate the OP is using, and whether the problem can be reproduced at a different rate.

I was using 44.1 and the metronome tip seemed to fix it. So weird.