Rapid clicking noises when trying to start?

Rapid clicking noises when trying to start then the car loses power. Put a jumper box on the battery light comes back give me a couple crank until it finally starts. Most of the times the crank be a long crank to the point the car comes on then when u let go it shut off. Replace the battery. Work one day perfectly. But recently I turn the key to run and it makes a rapid clicking noise (different from when I turn the key to run). I also heard gas from the gas line going back into the tank when sitting quietly. Put cables on the battery even though it new the car start letting me crank it, but the crank is weak. Then the car finally works.