08 GK Problem Unknown Help?

I just bought a used automatic 08 Tiburon about a week ago it has 37,000 miles and I'm already having problems (stupid dealer). When I'm breaking at about 1000 rpms the car/rpms "jumps" or revs once or twice then goes down. Now a few days later i feel vibrations on the gas pedal after about a few minutes of driving. Any help would be greatly appreciated.