nm 3.2 auto pajero going from 5th to 3rd

Hi guys I bought a nm diesel pajero 6 weeks ago that was looked after and drove fine it towed a caravan on trips, after about 2 weeks of getting to know the pajero I felt it go down gears on a down hill slope as I backed off it felt like it went from 5th to 3rd missing 4th. It did it again a day or 2 later and I went to manual shift to see if I was right about missing 4th and when I looked it was in 3rd. I Took it to auto transmission service centre they found some shavings in the box and said buy a new box, all transmissions get shavings I thought and the pajero doesn't hesitate going into gear , no shudder and changes smoothly , so I to it to another auto trans specialist they drove every where up down hills different speeds and their reply gearbox seems good its smooth doesn't hesitate etc ,that was on Thursday on Saturday went for a drive again down hill foot off the pedal it kicked down 5th to 3rd it over revs when you go to excelarate , it can also hold a long time in 3rd going up hills. I have read the ecu gets to know peoples driving and was wondering if the pajero needs a new ecu or to be reset no dash lights come on, or is it torque convertor anyone else had this?