A/t Transmission Problem

I Have A 98 Contour Se V6 2.5 Dohc. I Am Having The 1 - 2 Shift Bang. After Driving The Car For About 20 Minutes City Streets Or Freeway The 1-2 Shift Bang Happens. I Come To A Stop And When I Accelerate The 1 To 2 Shift Bang Happens. But If I Turn The Car Off And Restart It's Fine. Is This A Transmission Problem Or A Sensor Problem. Can Anybody Help Me?