Why does my 2006 ford taurus se with a V6 vulcan engine cut off and on while driving? mostly when a/c is on?

Started doing it once a week. Now every time I drive it cuts off and I have to shift to neutral and start again; or it cuts off ,then on when driving. Some times it cuts off, on, off , on . The needles plummet on the odometer, rpm and fuel gage.