forgot to add that if I'm setting  "Throttle (ms)" to 0 (zero) the system hang after a few seconds ..

Thanks.

I know what you mean that the switching off the throttle can cause too many serial messages and there is possibly to see similar output sometimes (but not so consistent)

That wasn't related to the throttle. If Visual Micro can discover the last upload information of a project in the xml files below the "[project_folder]\__vm" directory it will start debug when the serial monitor is opened. 

Debug uses the messages you see and when in debug mode it removes the messages from the normal serial view.

It's possible for an arduino to have a debug version uploaded via project A then to open a serial monitor on the same port with a different project or without a project. In the latter cases the serial monitor won't start debug and will show whatever it receives.

You will see the same after uploading a debug version in visual micro then using the arduino ide serial viewer.