You have more power than I do and I am having no issues even with the Cakewalk Tutorials without freezing, etc.
Do you have network connection via wireless? this has been known to cause issues.
I am assuming you have the latest drivers for your interface?
 

patm300e
You have more power than I do and I am having no issues even with the Cakewalk Tutorials without freezing, etc.
Do you have network connection via wireless? this has been known to cause issues.
I am assuming you have the latest drivers for your interface?
 

No I use internet via LAN cable. 
Yes I have latest updates of sound interface.
Must I change my sound interface? I think about changing interface but I want to be sure if the problem is from that

The audio interface drivers are the heart of a stable DAW, not so much how powerful a computer you have. That was 10 years ago. You have almost the same computer specs as me and I'm stable as a rock. So for sure I would think your audio interface drivers are not the best. 
You'll still be able to use your line 6 for recording guitar in WDM mode. 
But if you add a good interface with "known" stable drivers then you'll most likely be good to go. 
On this forum we have interfaces like Tascam, RME and Focusrite that are very popular and proof tested. There are others too but those are the ones I notice the most talked about.

Cactus Music
The audio interface drivers are the heart of a stable DAW, not so much how powerful a computer you have. That was 10 years ago. You have almost the same computer specs as me and I'm stable as a rock. So for sure I would think your audio interface drivers are not the best. 
You'll still be able to use your line 6 for recording guitar in WDM mode. 
But if you add a good interface with "known" stable drivers then you'll most likely be good to go. 
On this forum we have interfaces like Tascam, RME and Focusrite that are very popular and proof tested. There are others too but those are the ones I notice the most talked about.

So you advise me to change my Line 6 interface? I just wanted to listen that because I myself thought the problem is from interface, too.

Is there any other Line 6 people here who have no issues with Sonar?? ?
Maybe a good Idea would be to either start a new thread asking for feedback from other line6 users. I myself have not noticed any comments around here so dare not say it's good or bad. I just know what is working. 
I just don't see that it could be your computer at this point unless it's some crazy wreird thing like bad memory or a Video card causing havoc, anything is possible with computers. 

I'm curious as to why you have your VSTs on a seperate drive?

Cactus Music
Is there any other Line 6 people here who have no issues with Sonar?..

 
I use 3 different setups with 3 different Line6 interfaces. No real issues, apart from recent glitchy playback on my laptop, although I suspect that this is not interface-related (although I could be wrong).