Loss of accelaration

I take off in first gear and can put gas pedal to floor and it seems to big down until a certain point then it goes normal. It's the same for every gear. Also, I filled my tank up completely and drove maybe thirty miles and the next morning my gauge said I had half a tank. You would think if it leaked seven gallons of gas while sitting over night I would've noticed a smell of gas but I didn't. I just bought the car used and the previous owner had the wiring harness replaced due to wires catching fire. I am quite upset that none of this was mentioned before purchasing it. Please help. Thank you