make sure its the right plug. even brand specific. i had a boat with a goodyear outboard on it 35hp. any plug except the factory autolight plug would run great until engine was hot. then would slowly get squished closed at full rpm. wrapped my keychain ring around the bottom of it once to get off the lake. put in the proper ones never had an issue again.
check the top of your piston if you have a wrist pin bearing gone you may have put a dent in it when it was hot it will run good for a while then let go you can see it through spark plug hole or by removing y pipe