I solved the problem in this way:
I noticed that just before the focus was "stolen", the PC got "busy" for a second (beside the mouse appeared the small circle indicating the processor is working hard on sth).
So I started the Task Manager and gave a close look at the processes. I noticed that one process took over a relevat percentage of CPU usage exactly an instant before the focus was stolen from the task manager window.
Since it was a process that I don't need to be active all the time (sth about Vodafone Mobile connection, which I rarely use) I just stopped the process and then didn't allow the process to be started every time I turn on the PC. I did this last step using Piriform's free software "CCleaner", but I'm sure you can find different tools for this...
This was enough to fix it ...
Hope this helped!
Bye!