I've had the same problem in the past and the only thing that seemed to work was to have two projects on the go and with the same plug-ins and copy and paste each track to the new file songname 2, then saved the original and and the new folder and after ending Sonar done a computer re-start, loaded Sonar and songname 2 and for some reason that worked for me! I suspect the freezing/unfreezing of tracks could have caused the problem ... who knows ...hope it works for you.
 
 

Global bypass all plugins and see if the dropout persists. If it does, then you've eliminated plugins as suspects. If not, start deleting plugins one by one until the dropout stops happening.

bitflipper
 start deleting plugins one by one until the dropout stops happening.

Or bypass the FX bins and disconnect the synths one at a time.

Thanks guys for all your interest and ideas but I have solved it .... or so far so good.
 
I don't know how the new aux busses work exactly .. you'd assume, just like any other buss, but no, they are a little bit weird.
 
I had all my drums routed to an aux buss with Pro Channel compressor on it in order to parallel compress the drums, however, it wasn't activated. I discovered that engaging the Input Echo button would allow the song to play without dropout. Disengage Input Echo on the buss, and the problem returned. Is this not strange behavior for a buss?
 
In the end I just deleted the aux buss, put my plugins back, and all is well ... so far.
 
Cheers.
 
 
 
 
 

Glad you got it sorted

Wow, there's one for the archives.

Yeah, that was a nice find... one weird anomaly with the Aux tracks introduction that may/may not have been addressed was that soloing an Aux track muted it because the feeding track(s) also needed to be soloed. That was a bit problematic and confusing.