Can bad spark plugs be good for a little bit?

[Jackmanus]

The first time I took out the ski this year it was only going 38 miles an hour and I assumed that the spark plugs needed to be replaced but eventually it gave me full power so I thought it was all good. The next time I took it out it was doing the same thing only going 38 miles an hour. The fact that it eventually gave me full power for a little bit makes me wonder if the spark place could still be the issue. Thoughts?