What are some possible causes??

My 45 hp Mariner, (mid 80s 4 cyl) runs great at low to mid range but after half a minute or so of high speed running loses rpms. I can run on 1/2 to 2/3 throttle fine. The fuel filter and lines are good. Would a weak fuel pump cuase this? I dont want to change something only to find it was not the problem. Thanx