Let's start by saying there are a lot of possibilities. First I would see if I could bleed some air from the fuel system. If you have a small leak it can let the fuel bleed down while sitting and trap an air bubble in the lines. Once the fuel toward the engine from the bubble is used up, as in a little idling, the air hits and you're done till it is bled out.
2017 RAM DRW - sold
2020 RAM Longhorn DRW