2008 dodge 2500 shifting problem
 

Hi, I am new here and joined to see if I can get some help here. My son is in the Marine Corp in Afghanistan and I drove his truck up from Pendelton to Utah. It is a 2008 Dodge 2500 4x4 crew cab with a Cummins and a 6spd Automatic. he has done some mods to it such as EGR delete kit, Cold air intake and Smarty s67 catcher software. Here is the problem. Driving on the freeway it seems to lock/unlock the converter between 5th and 6th gear and then locks into 4th gear and stays there. I shut the truck off and it goes back to normal then does it again. The only thing I could do is manually shift it into 4th and as I got up to speed shift it into 5th and so on, but if I came up to a grade I had to manually downshift it or it would default back to 4th and stay there. Is there anyone who has had this happen to help me figure out what is going on? I would like to fix this before any real damage is done. thanks