Why is my car having troule starting and shutting off?

When I go get gas and the problem only occurs when i go get gas, the car has trouble starting after I get gas, It stales and shuts off. I have to turn the key and feed it gas at the same time. Also the lights in the car flicker and dim when I try to turn the key, is it the fuel filter? or something else? Also, when I put it in park and turn it off, then leave it in park and turn it on and rev the engine it only goes until 4RPMs and then shuts off completely, is this a saftey feature? It never did that before, before it would rev up to 9 RPMs and not shut off at all. I tired replacing the gas cap but that did nothing, plus the check engine light is on and has been on for about 3-4 weeks now. Help!