Misfire in a 95 peterbilt with cat engine?

we have changed all fuel injectors numerous of times, new high pressure fuel pump, and many other items. we would appreciate any input or suggestions on where to look next thank-you|||If it is a 3406c or d cat have the rack done on it. It sounds like a valve problem. If is not that have Cat check the pump timing and computer.|||bad valve is all i can think of, since these engines don't have spark plugs