Acts like it's slipping out of gear when accelerating

Have a 1984 Mercruiser 5.7L with Alpha one gen one. I accelerate slowly, nothing quick and get up on plane without issues. Once there, if I continue accelerating it acts like it goes into neutral. Rpm's instantly increase as boat speed instantly decreases. The prop is still spinning but much slower than it should be.Shift lever is not in the neutral position when this happens and the engine continues running. When it does happen I back off on the throttle, let it idle in neutral for a few seconds and easily take off again but once up on plane, it does the same thing with any additional acceleration. I recently had the propeller off when I installed a new impeller and water pump housing and I'm thinking I may have done something wrong putting the prop back on. What should I be looking at besides a new boat? Where to start and how deep I should go. I welcome and very much appreciate all suggestions

Have you noticed if the engine temp gets hotter than normal when running in the water?
Is so then it Sounds like it could be the coupler and if it is, engine must be removed to replace.

A coupler can go bad but still turn outdrive drive shaft at slow speed but will slip at higher rpm.

so at this point thats all i have.

Thank you kghost. I watch the temp. and there was no change. I checked the prop nut today and it was tight but I was able to pull the prop out and in. Seemed like about 1/2 inch of play. Is this normal or could it be causing the slip

Yes normal.

Not sure if you can do this,

Pull keys OUT of ignition.

Shift control into forward gear.

have drive trimmed down and level.
Spin prop Counter clock wise.
Once it is locked see if you can spin it by hand.
this may take a lot of effort but if your able to spin prop it is most likely your coupler.

Also the temps would increase atbthe speed at which you see the rpms rasing and boat slowing.
If drive is not spinning the there will be a lack of water being supplied as the impeller will not being spun.

So a lake test may be needed to trouble shoot more.

Most of the time when a coupler goes bad the engine runs but the drive, drive shaft doesnt.
You may be able see thevdrive shaft where it goes into coupler and watch if it is spinning like it should

Try another prop.