Loss of Power on '04 DF115

[oldfisherman]

I bought a used boat with an '04 DF115 Suzuki on it. The engine had not been run much and had RPM and low idle problems for a while. I kept running it and putting fresh fuel in it. The engine eventually started running great with plenty of power. Last week I checked the hours on the engine and saw that it was due for an oil change. I changed the engine oil and lower gear oil, checked the levels, and then ran the engine. It ran fine to start and then seemed to lose power. RPMs were fine but speed was down noticably. I checked the engine oil level after the engine cooled and it showed to be a quart too high. I drained excess off to full level. I ran it again yesterday and today and there might be some improvement but not to where it was. Any guesses?