So by lowest level you mean tabs are down more to push bow down?
The engine rpm will bounce like that on the limiter as it drops spark to cylinders until rpms come down.
If engine temp climbs at that rpm you need to look at the cooling system, if impeller has not been replaced I would do so. If impeller is in good shape would look to see if flow is restricted in manifolds elbows or ps cooler.
If you are getting 10% slip I think for these types of props/boating budget you are doing pretty good.
