I also have an Aqua Drive AND PSS shaft seal
. Since the double U joint "flexible" part of the Aqua Drive is only the part between the thrust bearing mounts and the engine
, the other half, that clamps to the shaft, should spin ONLY, and not move around AT ALL. If it does you could have a misaligned thrust bearing. This half has to be very accurately aligned on installation
, and should then stay that way. (Ours has for 15 years). If it is not that, and the shaft is not bent, Then I'd next check the play in the cutlass bearing. (1/32" - 1/16" MAX)
If it is not that either, then perhaps the bearings on the Aqua Drive are worn??? See if you can manually move the shaft up and down...