88 hp ghost rpm's

[jcturtle]

Hello, my son has a 1989 88 hp Evinrude. Sometimes, the engine will jump to full throttle when you ask it to, and bring him up on plane beautifully. Other times, it will go to about 2500 rpms for a few minutes ( it varies how long )
then it will go to full throttle by itself in a second. All the coils are working
at idle on the muffs. HELP!!! Where should we look for a fix? Thank you, John

[boats.net]

Start with checking fuel, you should also check the timing advance (make sure moving freely)