Honda 9.9 rpm issue?

[redbandit850]

I have a 2002 Honda 9.9 4 stroke that I have been using since new. Its on a aluminum at a family camp so I am not the only one that has access too it. During the past two seasons I have noticed something strange the engin has started to do. At certain points the engin seems to only run at half throttle and have a strong vibration. At one point I will be running at full speed and then the next seccond you can hear the rpms drop and the boat will vibrate a little bit and slow down. The slow down occures from 1 seccond to five minnutes. And then all of a sudden it will be back to full speed again. Sometimes it keeps going from full speed to half speed repeadly. This seems to happen when the engin is colder as when I am out for a whole day the problem seems to eventually stop. Not sure what could be the issue here. Im pretty sure this engin gets serviced at the beginning of every year though.