I have a Focusrite 18i20 and am on SPLAT, latest upgrade . All of a sudden, when I bring up Sonar it disconnects the interface. If I reboot, and e.g try playing a WAV file through WMP, it is fine. when I check out the interface with Scarlett Mix control, it is fine. when I brig up Sonar initially and check the audio interface in peferences, it is fine, but when I run the CWP, it becomes disconnected. It was working fine this afternoon. I then connected a MIDI cable from a Yamaha DTXpress II from the DTX out to the Scarlett MIDI in - after that it developed this strange behavior. I have of course disconnected the DTX from the computer and things are still awry. The other this is that when it  the audio interface has become functionally disconnected, and I have tried to play the project, the latency from hitting the on screen  play button till it starts rolling is long,

I'm completely grasping at straws here, as I don't own either of these devices. Did this possibly get set up so it's now using MTC or SMPTE instead of its internal clock( the 18i20, that is)?

Your I/F should be set to internal and SONAR to Audio Clock.
 
Does you I/F share a Hub with other USB devices like keyboard or Mouse?

1st or 2nd gen 18i20?  Is this a new development? (IOW, were you ever able to run without this problem?).
 
Make sure you have the latest firmware/drivers for the correct generation of your interface.
 
Make sure it's plugged into a USB2 port (Not USB3 which is blue).  If it's already in USB2, try a different port.

If it's first generation, do not use the beta drivers.  If you are, get help from Focusrite Support to uninstall them and reinstall the regular 1.8 driver.
 
If it's second generation, I don't know.
 

First generation - new development (have been using it for about 6 months, also have ADAT and SPDIF inputs going) - I have installed the latest drivers on their website.
 
None of the above suggestions worked/applied. Just chatted with Focusrite tech supportand he pointed me to a beta driver that he says usually clears this up - feels it is a firmware problem. I will try that when I get home later. Will keep y'all informed - thx for the suggestions - Ron