Acceleration is good but surges at any cruising speed

class c mh on E350 chassis, well maintained. while passing another vehicle on interstate at full throttle, engine suddenly lost power, went into limp mode; replaced all injectors, distributor, coil and condenser, MAP sensor, fuel press regulator, high press fuel pump, fuel filters, plugs and wires, fuel pump relay, throttle position sensor, fuel line hoses; egr is functioning normally, O2 sensors good, transmission and shifting sensor no problems, engine timing is correct, compression good and consistent on all cylinders, no problem with catalytic converter and there no diagnostic codes, no engine light, fuel press is correct. engine accelerates well but surges at any cruising speed. throttle body has been cleaned, air intake system is clean and intact, no vacuum leaks. Please!! I need help and cannot find any local tech familiar with this vintage vehicle.460cid, efi,eod parameter hunting