1999 Mercury 200HP EFI Outboard

My mercury seems to be losing its prime or something as far as fuel goes. I ran it this weekend and a couple of times while driving the boat the engine would stop. I would hit the little bubble and prime it again and be off and running normal. It only done that like two times while riding and not towing a skier or something. When I attempted to pull a skier that could not get up with ease. The boat seemed to lose its prime worse. Maybe it was the frequent stops and starts. I could hit the little primer bubble and be up and running again. I have noticed this before last summer but not real frequent. I thought maybe the engine loses its prime from time to time. This weekend got to be a bit more frequent than I want to deal with trying to tow someone. Someone give me advice on what I may can replace or clean to see if I can stop this problem.
Thanks