I have take a picture so you can see the distance of the prop.
if you turn the prop so one of the blades is top - what is the clearance (space) between the blade tip and the bottom of the cav plate? YOu might be suffering from the same problem as I did when I bought my boat.
It might be an illusion from the angle of your picture, but I had a similar thing to what you describe on a Yam 55 which came with my boat. - Yam sold a prop called a a "speed" prop - it had reduced diameter for less prop drag, so the engine's power could go to spinning a higher pitch to push the boat along faster.
That theory worked fine with a light weight flat bottom "skimming" hull, but I had this on the back of a 5m 1/2 ton rib!
The prop didnlt have enough blade area to "grip" the water, so it let go like you describe when I opened the throttle too far. It also let go at every third wave, if I turned too quickly...
What number is stamped on the prop? Normally that will give a clue to diameter as well as pitch.
I had a similar (bit not so spectacular) problem when I put the Merc on the transom. it had a 10.5" dia prop - 14" pitch. Over revved nicely (and let go if I pushed it hard in big waves.) replaced with another 14" pitch but 11" dia. It now never lets go, and the engine feels like it's connected "solidly" to the water.
All of that because the engines were on ligher / less drag inducing boats before I got them.[/QUOTE]