Engine power loss after driving about one to two hours

No check engine light is on, after driving about an hour or so the engine will start to loose power, the speed will go from 75 mph 70, 65, 60 etc, when the engine starts to loose power if you put the car into neutral and turn the key off and let the car cost for about ten seconds and restart the car it will pick up the engine power again for about fifteen minutes then start to loose the power again, when the car acts up if you try to speed up it will start to buck, and act like it bogs down. Things I have tried to do to correct the problem was change one catalytic converter, removed the Mass air flow sensor and cleaned it with mass air flow cleaner. Thanks for any comments.