Boss Elon Musk said Tesla Inc is tweaking its self-driving software to eliminate a phantom braking problem and may release a significantly improved version within the next couple of months. Tesla shares were down 4.4 percent at $589.89 in the late afternoon, extending falls after the U.S. safety regulator said it has launched a probe into a fatal crash near Fontana, California, involving one of the company’s vehicles. U.S. federal and state regulators have been scrutinizing Tesla’s semi-automated driving system following accidents in Texas and other states. In March, Tesla told California regulators it may not achieve full self-driving technology by the end of this year. It said it is currently offering a driver assistant, level-2 technology that requires driver supervision. “I think we’re maybe a month or two away from wide beta. But these things are hard to predict accurately,” Musk said in a tweet on Wednesday. In April, …