1996 Other Chevrolet Models Spits back through injector body

I have a 96 Motorhome with a chev 454 , fuel injection. It spits back through the injector body when I step on the gas hard. rough idle and high speed miss. I have replaced the rotor,cap, wires, and plugs. checked for vacum leaks and found none. cleaned the air filter,it has a washable element. I checked for codes and there are none. I did just fill up the gas tank, could it be bad gas or dirt in the system?