Hey Fabrizio I've find out the following:
-Connect the USB cable directly to the computer, not trhough a USB hub or anything like it.
-If you are using the virus you might want to reset it meaning go to the beginning of the sequence so that everything can load from scratch. Usually what happens is that some midi messages are left hanging and this can cause your virus to be out of sync. You can also try hittin on the PANIC button just to make sure.
-Another thing I have notice is that if you are trying to do many things, say for example your sequence is playing and you are changing patches it may cause the virus to get confuse and break.
They way I usually get around it is I just use my virus as a multi timbral instruments and have the sound coming out though the audio outputs it never chokes and you can control it via midi just like a sound module. If you want to automate something then just assign the CC#. I think is by far better and you will have a better experience like this. Besides the knobs are there for something no?