my old nissan is showing its age

i have a 90 hp 1988 nissan outboard that has begun to bog down after 10-12 seconds on plane. it runs great at the hole shot, but as noted above, it drops to idle speed after ten seconds or so. after the rpm returns to idle, it can usually be revved up for another few seconds. it will usually stay on plane longer if i just barely get it on plane and then pull the throttle back to 3000 rpm. any ideas?

Change the fuel pump diaphragm and all fuel lines. Time for a good carb cleaning too.

someone elsewhere suggested that the fuel tank vent line might be clogged. it certainly acts like it is could be either problem. our mud daubers have found lots of mud to play with this year. i'll check it first as it would be an easy fix. i found a source for the fuel pump diaphragm too, so that fix sounds doable too. thanks.