As you describe above fault could be in electronic shifting system ( on positioning a gear lever at different positions P,D,R Electromagnetic valve opening / closing / changing ATF flowing path ) change ATF regularly as per manufacturer specifications & clean / inspect transmission filter, could be strainer type OR Magnet screen filter, magnet collecting a all metal pieces / debris flowing with ATF, so we can guess what's happening inside transmission.if transmission performance going down / low pulling etc after some 1-2 hours or less then hour then first change ATF if this will not work thats means transmission needs repair ( transmission oil pump not working properly,internal oil leakage from torque converter or gears Assembly / groups problem transmission disks etc.