Q: Car is completely dead but comes back on after a few hours. But in about a week it dies again. And won't jump.

Bought a used car from Kia dealership. After about 4 months of driving this car, it randomly died. Can't unlock it, no lights come on, nothing happens when you stick the key in the ignition. After a few hours, car started without any issues. Worked normally for about a week. Then dies again. Immediately tried to jump it, and nothing happened. After taking the jumper cables off of the working car (while connected to my dead one) and tapping the ends together, the lights came on but still wouldn't start. Tapped the ends together again, car started. Took the car to the dealership, they ran a diagnostic and said it was the battery and needed an oil change. I replaced the battery with a brand new one, but had JUST HAD an oil change few weeks prior. So i skipped the oil change. Car ran great! Then a week later, dead again. After a few hours I, went back to the car, it started. Took the car back to the dealership, they kept the car for a week and said they could find nothing wrong. Help?

My car has 62000 miles.
My car has an automatic transmission.