vn 5.0 probs

i have a vn 5.0l on duel fuel. it idols and runs ok on petrol and gas up to 3000rpm. but as soon as it hit 3k the motor cuts out to a idol and wont rev any more! it sounds like its computer controlled i have done the human check on the ecu and it doesnt show any problems. gone over the motor no vaccum leaks cat is clean. can any one help!