My 99 Pontiac Grand Prix with automatic transmission started to stall this morning. When I stepped on the gas pedal, it would take a few seconds to actually take off. Just a few minutes ago on the freeway, I could not get it to go more than 30 mph and I had the pedal to the floor. When I got home I got in the car and changed the gears starting from 1 to D and it went over 30 mph. What can be causing this??