98 SN prop spin or transmission

Got a problem with take off at heavy throttle. RPMS grow but the acceleration almost immediately declines. The boat has the inital jump but almost immediately loses bite. If i take off very slowly the boat will get up to full speed with out the issue occurring and will run wide open. I also think i hear something spinning when the issue occurs. The tranny fluid is full and looks fairly clean. Any ideas or thoughts. If it is the transmission what do you think the cost on a rebuild would be?

Check your tranny fluid, maybe you have a slow leak and you are low.
If this is the case, change your fluid now to get the burned stuff out.

Also check the prop to make sure your keyway has not sheared.
If your prop was never lapped in the only thing keeping it from spinning is the keyway.
I have never seen this happen but you should rule it out..
Take the nut off your prop shaft and inspect.

Thanks for the assist Pete.
I have replaced the clutches in the 1:1 velvet drive twice in past 30 years. It is not too hard if you can turn a wrench. Both mine worked after the fix.
Have not opened up a 1.23 to 1 gearbox yet. Hope I never have to.