Hyundai Santa Fe LX 2004 auto transmission problem

Hi, 46000 miles/v6/ awd
problem is around 20 mph when driving normal NO heavy foot, it slams into gear, suv jerks, but when I watch the gauge and go alittle slower at 20 pmh it goes into gear fine, at no other time does this happen. Had it brought in reset whatever they reset and change fliud, lasted only one drive, its doing it again!
thank you for your help,
psI sent a sm donation but Will come back with another donation if the answer is helpfull, thank you for this site!