I would probably try to uninstall, and reinstall drivers again (just to make sure everything is up to date). Do you have any opportunity to update drivers via the ux2?

Long ago, I ran with Windows and Sonar fighting over the ASIO drivers for the audio interface, and even after disabling Windows sounds, it was still problematic, so I finally came up with some different ways of doing things, and have been much happier and audio issue free since splitting things up.  (whatever application has a hold on the ASIO drivers would prevent other applications from seeing/using it too).
 
Option 1 I came up with:
 
1.  Set Windows Default Audio Device to the on-board sound chip on the motherboard.
2.  Set Sonar and other stand-alone music programs to use the audio interface with ASIO.
3.  Plug a set of computer speakers into the back panel of the computer for Windows sounds.
 
No problems with above, but computer speakers were pretty cheesy.
 
Option 2
 
1.  Set Windows Default Audio Device to the on-board sound chip on the motherboard.
2.  Set Sonar and other stand-alone music programs to use the audio interface with ASIO.
3.  Split signal from computer speaker jack into a left and a right quarter-inch cables, running them both into inputs on my audio interface, for much better sound.
 
No problems, but it consumes 2 inputs on my audio interface.
 
Option 3 - and what I currently use
 
1.  Set Windows Default Audio Device to the HDMI connection running to one of my 2 HDTV's that I use when running Sonar.
2.  Set Sonar and other stand-alone music programs to use the audio interface with ASIO.
 
For my situation, this is the best balance of it all.  HDMI carries both audio and video through the cable, and since both of my 2 displays are actually HDTV's, rather than standard computer monitors, either one's speakers can be used, and have decent enough sound for YouTube or Windows Media Player sounds.
 
My 2 HDTV's were super cheap and fabulously huge (40" and a 32"), which helps my 55-year old eyes see things better.  I got the 32" for $198, and the 40" for $219.
 
So, I never have any ASIO conflicts, and no particularly challenging cabling is required, and it all works, all the time.
 
Bob Bone
 
 

Bob - I use your option (1) with the addition of a monitor matrix.  No cheesy speakers ... + some other bennies

SevadaX6
I haven't used Sonar X1 , but the problem is like what you say. I must kill SONARPDR.exe in Task Manager and then reboot in order to use ASIO again.

 
Interesting...
 

SevadaX6

Sometimes when I work on a project and for example open a VST, my Sonar X3 doesn't respond

 
Is it a particular VST?

robert_e_bone
So, I never have any ASIO conflicts, and no particularly challenging cabling is required, and it all works, all the time.
 
Bob Bone

What if I enable onboard sound card for Windows (without any speaker for it) when I work with Sonar (or any DAW) and use ASIO just for Sonar, and then when I want to use Windows programs (e.g. winamp), close Sonar and enable my external sound card for Win ? I must test it
 

synkrotron
Is it a particular VST?

For example when I use "Action Strike" in Kontakt, and then try to save the project, Sonar doesn't respond

FWIW, I am currently using my ASIO QUAD-CAPTURE as my audio device for everything. Do have my windows sounds turned off though, so no bleeps or clicks from the OS are sent to my Q-C.
 
I am currently doing a lot of switching between a Sonar session and then listening to media on the internet, and I won't even close down Sonar. I may even listen to something saved as MP3 on my hard drive, with Windows Media Player, while Sonar is running.
 
It may be considered to be bad practice but I never have any problems whatsoever.
 
The only time I close down Sonar is when I use something like Sound Forge, or another tool that demands exclusive use of the ASIO sound device.

I have had this problem a lot.  Sometimes it was caused by opening a soft synth in standalone while Sonar was open (or vice-versa).

I posted what I had come up with for choices that work for me with zero issues.  I had issues prior to that when trying to have everything go after the ASIO drivers of the audio interface at the same time.
 
I hope that if someone is having trouble, and they find one of my options works for them, that they are able to move forward.    :)
 
Bob Bone