94 SS Xi JH750B wouldn't rev over 3600 rpm in the water

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My 94 SS Xi JH750B wouldn't rev over 3600 rpm in the water yesterday; limiting it's speed to about 1/4 of it's max speed. Once home and hooked up with a flush kit, I was able to get it to rev higher, but anytime it rev'd over 4000, the RPMs were very unstable and I could not control the RPM's. With the throttle held steady, the RPM's would fluctuate wildly.

Any ideas?

Many thanks in advance from a site newbie!