Gearbox problem first Chrysler

Had the car 2 days it’s a 58 plate voyage 2.8 auto , fantastic looking car , but now we have problems, hit the motorway , went straight into limp mode won’t go over 50 mph , foot to the floor 4000 revs and nothing but a crawl , had to get recovered by the aa for my 200 mile trip home , anyway I’m still waiting on the full report I do remember him saying gear ratio Mis-match of something 3 upwards , been to the garage to have oil checked, though it might have been dirty , it was brand spanking new , but the gearbox is still , pissing about , it’s our first ever Chrysler , so don’t know anything about these cars , does it need a new gear box ? , the car has 140,000 miles on it and at some point it’s had a change of gear box but I don’t know the history, sorry for the long post , thanks