01, 3.0 miss/backfire at 4g in gear

01 Ranger 3.0 V6 auto. Truck runs fine at idle runs fine going down the road unless you need to accelerate quickly. Runs perfect until you hold it to the floor and the tach reaches 4g then it starts acting like the rev limiter has kicked in. Never had a problem with it until a few weeks ago and it started doing this. Took it to a mechanic and he put new plugs and wires on it and claimed it was fixed. Before I took it to him it would do the same thing but at 3g under load.