Power Assist Brake problems of the 2001 Volvo V40

While backing vehicle out of garage in morning the brakes sometimes become very difficult to push (hence, difficult to stop the vehicle). Prior to very recently, the problem only occurred while backing out of our garage down the driveway (1-2% grade, only). Recently, the problem is beginning to occur while traveling forward, again it is only when the vehicle has been sitting for hours and just started. I have yet to call Volvo. The problem has been so intermittent up to this point I know what will happen: we will be without a car for a week while Volvo drives it and finally reports to us that there is no problem. Have you folks heard anything on this peculiar problem???? dt.