We had this problem too, and after many visits and arguments with multiple dealers and Hyundai complaints department, transmission service, many sensor erplacements, eventually convinced dealer to swap the transmission computer with another car in the yard and the problem disapeared! Has been OK for over 12 months now, but don't say it too loud
PS the transmission computer has a fail-safe mode taht locks the transmission in 3rd, whter you stopped at lights or driving at higheay speed