Well it seems that the powered USB hub has solved the problem. 
 
I've been opening and playing projects this evening, and using the A500 controller and everything appears to be working perfectly. I can only assume that the sound interface was too heavy power-wise for my laptop's USB ports, and adding the controller just made it worse.

I'm glad the problem is solved, but this underscores the importance of listing the gear you use with quite a bit of detail. Non-powered hubs are universally considered pretty much unsuitable for audio applications, and your experience tends to confirm this. Maybe this needs to be in SONAR's documentation or the TASCAM interfaces going forward.
 
In any event, hopefully you can have fun with your gear now!

Ha  ha and now for the next month you'll be asking everyone with issues if they are using a USB hub! I wonder what the list of troubleshooting tips is anyhow, the documentation is really not obvious here. Like my tip on always checking your DPCLAT  with Latency Mon , that should be in big print I think. 

johnnyV
Ha  ha and now for the next month you'll be asking everyone with issues if they are using a USB hub!

Yes, I'm adding it to the list 
 
I think there should be a pre-flight for computer recording like there is for airplanes.