15HP motor problem
Sounds like a fuel delivery problem, likely the carb or the bulb.
When your motor starts to die, is the bulb firm or squishy? When the motor begins to die, pump the bulb and see if it makes a difference.
If it isn't a bad bulb or diaphragm, it is likely a carb that needs to be rebuilt / cleaned or both.
80% of outboard issues are fuel related, for older motors like yours, I'd guess 90%. They were never designed to run with ethanol - it really does a number on them.