Have a 2008 Elantra GLS with 63K miles. A/T was serviced by dealer at 51K. Car now occasionally downshifts from 4th to 3rd when driving between 60 and 70 mph. Happens only when trans is hot (after 2-3 hours on hiway). When it occurs, it searches for the right gear, going back and forth from 3rd to 4th for several minutes before returning to normal. One thing odd is if I shut the car off for 5 minutes after it occurs, it runs fine afterwards. Do I need an ECU update or do I have a A/T problem?