Problems with the automatic Kia Picanto gearbox

Good morning, my mother bought a brand new 2007 Kia Picanto from the dealership. It seemed to be running well until about a year ago when it started having a problem. Sometimes the transmission seems to stick or lock up, and when starting from a stop, it makes a loud clunk. It only has 100,000 km on it. I would appreciate your help, as she has taken it to several mechanics who tell her that these cars often have this problem and that the transmission needs to be rebuilt.