optimax question

[pair of jacks]

I have a 2000 Optimax 200hp that very recently has developed a quirk. After I run for 3-4 miles say at 25 kts. it then drops down to say 22 kts for a second or two then back up and keeps repeating- down then up. It starts right up and idles great and I can troll all day without a hiccup- but on a sustained run over some miles, this "surging" occurs. I have no alarms or warning lights lit. Also, if I stop and fish for a while, then start it, it won't do it again until I run for a couple of miles. Any clues as to causes ? Help !!