Wow, this is an old thread. Did you get it fixed? My first suggestion is a new set of spark plugs. When my engines performance drops off I put in a new set and boy does it make a big difference.
I have a '96 Speedster. The only time I've had problems like you describe is when there are weeds blocking the intake area. I'd take the boat out of the water and check that grate area very well. I don't think a shaft moving fore/aft would have that problem. When you say you've checked the wear rings and they are okay, does that mean you pulled off the back ends so you could actually see the impellers?
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