91 Johnson 40 HP

[dwagner]

Motor starts good and runs fine at first. After an hour or 2 of on and off use the motor slows down then speeds up, up & down, up & down. Stays running but sounds like it is going to die. Another time it stayed steady but couldn't get any speed out of it.

[boats.net]

Try pumping the primer bulb when it starts to surge. If this eliminates the surging for the moment, then could be the fuel pump.