Bought used from Hyundai dealer 20K miles on it auto tranny all the bells and whistles supposedly a leased vehicle turn in. Test drive was great no obvious problems. Put about 500 miles on it then noticed when at normal operating temp with cruise control on, flat road no hills, it would surge in and out from 5th to 6th gear. Not constant thing maybe every 30 to 60 seconds. Did it on quick run to grocery store. Next day going to work did it more frequently 30 to 40 MPH more than 50. Got home called dealer service dept he said bring it in and they'll put it on the computer see if it needs reprogramming. I asked if this was a common problem with their auto trannys, of course he said no. OK, I'll be there at 8: AM Monday.
It is a Hyundai certified vehicle so won't cost me anything. I believe their certified vehicles are required to go through a multi-point inspection and I should imagine checking the auto tranny fluid is one of them. However, I check the air filter and it was so dirty you could barely see light of day through it, so their multi-point inspection team gets a zero for the day.

Let's back up to where I said "when at normal operating temp". Yesterday I paid special attention when it was cold it did not do it. After driving six miles then it sat for 20 minutes I got back in it and it did the same surging thing. So my conclusion is, it does it only when at normal operating temp.
Daytime temps here have been unusually hot, 105 to 110 heat index. But that should not make it surge like that. I have a garage and see no tranny fluid on the floor. So tomorrow AM we'll present it for a exam and see where it goes and I'll report the findings back to you good folks.