96 xp will go 25 mph and then take off to 50 till i let off then back to 25

[Jerimy]

I have a 96 xp i just replace the impeller and wear ring and bearings in the back end. The motor runs great out of the water one the garden hose, but under load in the water it will only go 25 for half the lake then takes off like it should but if you let off its back to 25. I pulled the plugs and they are both the same color and i cleaned the exhaust valves which also didnt help, compression is the same on both cylinders. Any ideas we are running out of summer days in minnesota...ha ha Thanks