Folks,
Oftentimes when I copy paste in PRV, using Ctrl-Drag, notes get dropped kind of randomly, as if I had let go of the mouse button. I have to undo and try again, and get similar results. It's a little frustrating. Is there a limit to how many measures one can move? 
 
Windows 10 and Sonar Platinum.

I Ctrl-Dragged notes to about 550 measures from the original  No dropped notes here.  Using Platinum early release 2017.1

it could be overlapping clips in the midi track.
when one clip is behind another clip (they are not one whole clip) selection can be difficult because sonar does not know what clip you want selected. My advise is to bounce to clips your midi track as often as possible when selecting / drag CNTL copying between to midi clips. - Also note: Don't let go of the CNTRL button until after you have dropped the notes.
 
Also if you are using groove clips, bounce them to a normal midi clip first before dragging CNTRL copying.
this will eliminate more dropped notes on the paste.
 
 

Don't discount the possibility that you have a faulty mouse. For instance, some Logitech mice have a tendency to develop a "phantom click" problem (Google it) which means that oftentimes an extra click will fire off with the first one. This can be an absolute nightmare if you're dragging things because that second click aborts the drag. Last time I had that happen with a Logitech mouse it made using Sonar (indeed anything) a very frustrating experience indeed. 
 
You can check if your mouse has a phantom click problem by going into the mouse options in your control panel and trying out their double click tester. Control Panel -> Mouse -> Buttons -> Double Click Speed. There is a dummy folder icon there you can use to test the double click. Fire off a long succession of single clicks on that icon, with a gap of perhaps a second between each one. If at any time that folder icon opens, you know you have a double click problem. 
 
I guess there also exists a small possibility that you have a faulty CTRL key too. I used to have a keyboard where the CTRL key wasn't reliable - sometimes I would have to press it extra hard - and that made everything a nightmare too.