Merc 50 hp loses power at wot

[Randy Rowlette]

I have a merc 500 50hp serial #:3239179. The motor runs great when first started. After 20 to 30 minutes of running time, it will not stay at full throttle. It acts like it is running out of fuel. It runs great at an idle but once you give it throttle it will run for a while then die back to an idle. It never quits running, just won't maintain any throttle other than an idle. I have replaced all the fuel lines and replaced the fuel pump diaphram about a year ago. Any ideas, I'm stumped.