I have a 2003 50 hp mercury outboard. I just had the water impeller replaced. When i first take it out, it runs great, but after the motor warms up it starts surging. Once the boat planes out you can feel it surging. Once it planes out its running about 5200 rpm and about 38 mph. I can stop then try to take off again and it just boggs down at about 3000 rpm. It wont take of, wont plane out, just has no power. I can let it sit for about 20 minutes then it runs fine for about 5 minutes. The plugs are fine. Now the boat repair place that replaced the water impeller told us to make sure we mix in oil with the gas. So before i took it out i mixed in 28 ozs of 2 stroke oil with 10 gallons of gas. Has anybody ever experienced this, and do you know what could be the problem. Thanks for any help.