1995 Mariner 75 hp...WOT 3000 rpms

After several years of not owning a boat,my boys are at the age that they wanted me to get one and I agreed with them...soooo i found a 1995 key west 1720 sportsman with a 1995 mariner 75hp....i went and looked at it it looked very clean and was about what i wanted so i took it out on the lake and ran it hard for 30 min or so....could use a bigger motor but it was adequate for what i need and was a reasonable price compared to other boats i was considering...and it ran great.Next i checked the foot for water and found none...checked compression 120-125 psi on all 3 cylinders...awesome...I bought the boat and hauled it home...The next weekend I took it out just to learn a little more about the way it handled and how it started with the engine cold,I wanted to do this before i took an 11 and 5 yr old so if there was any problems it wouldnt be a big deal.So i stop at the gas station and put 10 gal of 93 octane non ethonol gas in it..the boat fires right up and idles beautifully,so i ease out out of the cove and hit the throttle,the boat came up on plane and is running just as when i tested it....for about a minute...the the rpms fall to 3000 and will not go above that with a load on it still idles and starts fine and the engine runs smooth...I am taking the boat monday to have the impeller replaced and i guess ill have them try to figure out what the problem is then unless i can before then..the only thing i did to it was added some gas...any ideas what my problem might be?

make sure the timing is advancing properly...