Sorry for the rookie question. Any help is appreciated.

I have a Mercury 25Hp on my pontoon boat. Recently, when first
starting out I can push it full throttle and it will only go about 1/4
of the speed it usually does. After a while it will then go full
speed for about 20 seconds then back down to 1/4 speed. All this
while the throttle is pushed all the way up.

I switched fuel tanks to no avail.

Again, thanks for any help in advance.

jason