Alpha slippage
If the propeller hub and drive coupling have been ruled out there are 2 other things things that may be causing the problem.
Have any through hull fittings or transducers been added near the transom close to the keel?
The most likely cause may be where the propeller fits in the gear case where there are a series of raised rings which make a labyrinth seal to keep exhaust gasses from causing “ventilation”. If at one time the propeller was installed without the thrust collar that can wear out the rings as well as a hub that may have failed and the propeller became off centered. If you have an exhaust bellows that goes from the gimbal housing to the bell housing try this. Raise the drive and cut the bellows about 3” from the gimbal housing and remove the rest from bell housing. If you already have the short exhaust smooth style then you will need a new gear housing or add through transom exhaust.