Engine surged, then wouldn't go into drive.

Hello all- My new to me 2010 Elantra Touring car started dying at red lights. Then it sometimes died from park to drive after dying at the signals. I thought it needed spark plugs? and recently the engine started to race. I pulled over and stopped, started the engine again and drive wouldn't engage. I stopped engine and started again and the car drove fine for 500 feet and started a repeat of the previous problem. I called a tow truck and followed it 40 miles to my mechanics shop, which had closed for the evening. Before leaving the shop, I started the vehicle and drove forward a few feet and then the transmission wouldn't engage into reverse. I was hoping it was the throttle pressure sensor that I read about while waiting for the tow truck but my mechanic wants me to take it to a Hyundai dealer as he has no idea what it is and is thinking transmission. My little car has 151 thousand miles on it and has been well taken care of by the previous owner. Any ideas? Thank you.
Also, no engine lights came on.