I have a 1997 merc 150 XRi. It has run seamlessly for the last 6 years since I purchased it. I always fill the tank and put additive in the fall, and give it a shot of Sea-foam in the spring. This summer when I put boat in water it won't maintain a consistent speed at higher range. My rev's go up so far and it holds. Everyone once in awhile it lunges forward and down. I have always run premium fuel and did nothing out of the ordinary in storage last year. The lunges are intermittent and when it does only for a second then bacl down. Any ideas??