My project has 32 tracks, 6 midi and the rest audio. I have 2 tracks using TH3 amp sims Cakewalk Edition and it seems to me the issue started when I added those but can't be 100%. 
 
In an effort to fix this, I have bounced all tracks to single clips and frozen the 2 TH3 tracks. So no real time effects active right now. 
 
The short hang takes about 15-20 seconds before I get control back. it's teaching me patience. :) It seems to happen when I click on something (a clip, a midi note, or even the horizontal scroll bar), whether in the Track View or Piano Roll. 
 
When I check Task Manager/Performance, I see no memory, CPU, or disk spikes at the time this occurs. It seems to be Sonar doing some background processing. I have set autosave to every 20 minutes or 20 updates so it's not saving. In fact, the usual progress bar in now-time display doesn't appear when I get the hourglass. 
 
Finally, Google searches of Sonar, not responding, and hourglass seem to be limited and always related to project loading. No issues with playback once I am unfrozen and can actually hit Play. 
 
Thanks for any recommendations you all may have. 
 
Steve G
 
Windows 10 64 bit - Sonar Platinum - 8 GB Memory - 600 GB available disk

do you have plug in load balancing on in preferences ?
If not, turn it on, if you do, turn it off. Take note of the changes. I know someone else who was experiencing difficulties with hour glass and this help significantly.

You can load the song in Safe Mode and select cancel when Sonar asks if you want to load TH3. If it opens clean, save it. Then try to load 1 new instance of TH3. Test closing and opening  - rinse repeat for  next TH3.

Thanks folks. It is definitely TH3. I haven't yet finished the Safe Mode route (see more below), but did get the issue to go away by deleting TH3 from the FX bins. Voila! 
 
Thanks also for the load balancing idea, though that didn't fix this one. 
 
Regarding Safe Mode - I got tired of hitting Yes Load/OK as I apparently have MANY instances of Tube Saturation, EQ, and Compression on "Untitled Tracks". These sound like Pro Channel presets I have used/ and I did copy this project from a previous song and may have brought a bunch of garbage to it. 
 
I guess I am "Solved" but would be interested to know where these Untitled Track Pro Channel plug ins are coming from. 
 
Thanks again for helping me solve this!