kday
 
I'm using VS-700 and Windows 7 64bit

And this should have been the first thing you mentioned.
 
the VS 700 has an ASIO driver for Windows 7 that you should be using.
If you are already using that Windows 7 driver for the VS 700 then I suspect a hardware conflict, driver conflict to be the culprit. Make sure your video card is updated (even if its onboard) and look for windows updates. Sometimes MS Visual studio, C++ libs get out of date and sonar uses latest versions.

Just a long shot , but there are demo plug ins that cause audio to drop out until authorized. 
 
But as said, update windows, Run Latency monitor and use the ASIO drivers for sure. And best you always use the interface when running Sonar. 
 
http://www.resplendence.com/latencymon