2005 Pontiac Montana Car dies when at an incline

This started a few weeks ago, I had a half tank of gas in my car. I was parked downhill waiting for my son's bus, the gas hand went to E and the car died. I had to roll the car to flat ground before it would start. Then today I was parked at a slight incline downhill and it died again. The gas hand goes wacky on me, it goes up then down, it doesn't really stay steady. Now I realize the hand will go up if on a uphill incline and vice versa for going down on downhill incline. But this things goes up and down even when driving on the highway. Sometimes accelerating will lag then lurch forward like a backfire or like transmission jumping into gear.