paccar mx13

Hello im not sure if this is the right place to be asking this but I have a 2013 388 peterbilt with a paccar mx13 eaton fuller 13spd trans. I have been having issues when loaded as I get to my last gear truck seems to go into limp mode is the only way I can describe it. So when it does it, I come out of gear and go back into gear and it runs normal and it only does this when loaded I haul grain in Kansas. Just wondering if anyone has had or has this problem and found any solutions or point me to the right direction on hopefully finding the problem thank you!