So are you saying the dealership cannot find the problem? Did they only check for codes or give your motor a good going over?
I had this same issue with a 97 Ford that I owned many years ago and the dealership tried unsuccessfully to find the problem on two separate occassions. They finally hooked up a data logger to my OBDII port so they could "see" it happen as I was driving over a 1-2 week period. Not sure if Hyundai has such a device but I would think they may. Basically, when ever it happend while I was driving I would press a red button and it would log
30 sec back in time and 5 minutes moving forward so you could see exactly what was going on.
Ended up being a bad O2 sensor that had gotten "water logged". Once replaced, everything was back to normal and I was a very happy camper. Not sure if this applies here but it sounds like your looking to turn over every rock you can for a possible solution.