mercury 9.8hp problems

[misterleon]

I have an early 80's 9.8 mercury outboard. The problem I'm having is it only goes to about 20% power. It sounds fine at first but once you reach a certain point on the throttle it revs real high and slows down and just keeps repeating this. I checked the prop multiple times to see if the hub was bad with the line test and its been fine. I've also lowered it further into the water in case it was sucking air into the intake but it still did the same thing. Mercury still thinks its the prop, but I don't want to spend $110 if thats not the problem. Can anyone think of anything else it could be?

Thanks