Allison jumps when shifting into 4th spd

[Eddio350]

Hi Folks, This is my first post on here. I am new to Rv-ing having been a boater all my life, but now I have a 1999 American Eagle EVS40. It has a 350 Cummings and a six speed Allison, I get a jumping or bucking feeling at about 40 MPH, I changed the fuel filters because I thought it might be starving but it is still doing it. Could this be the transmission? Maybe the computer? when it happens the tach drops about 100 rpm. it has no power on hills, can any one give me something to look for or some ideas. I'm and gas engine man and don't know to much about diesels or even gas engines newer that 1980. any help on this is welcome, thank you. Edd