2001 Other Hyundai Models Transmission inconsistency?

I have a 2001 Hyundai XG 300 std sedan, with 105,000 mi. Overall, it has been a good car. At 72,000 the tranny 'shuttered" Dealer replaced the speed sensor to no avail, flashed the computer, then replaced the enitre tranny. For some time now, intermittanly, the tranny will "buck" when starting off from a standstill (as a new driver does learning a stick shift) or at times, in a 'hurry' start the car does not immediately respond, then kicks in too fast. I then have to put the car in nuetral, rev the engine, start again slowly and then she seems to shift fine until the next incident. I have narrowed this down to not letting the car idle for at least 30 sec or more but sometimes idleing too long makes it happen as well. If I go to the dealer, they replace the speed sensor. For $116.00, but nothing changes longterm. Is there a computer issue, a sensor issue, or is there something else? Help !