From 84 onwards, all automatic 928's are susceptible to Thrust Bearing Failure, as the torque tube creeps in the front clamp and the flex plate applies preload to the thrust bearing surface at the center main bearing.
Prior to 84 the torque tubes were machined for, and had fitted, a circlip which prevented shaft migration (although if those cars have been serviced there's a chance the clips were left off in which case you're prone too).
Your 86 auto is susceptible - the "fix" is firstly to measure the crank end play to make sure the thrust bearing is still within the factory-stated acceptable wear limit.
The spec for a new thrust bearing is between 0.06mm and 0.19mm. The wear limit is 0.4mm.
If you have more than that then you should consider either separating the girdle and replacing the bearing, or buying a spare used engine while they're still available relatively cheaply.
After measuring to make sure your bearing isn't worn, it can be prevented by regular maintenance of releasing the flex plate to release any preload that's built up since last service, and undoing and re-torquing the rear torque tube coupler at the gearbox, followed by retorquing the front clamp.
There are also couple of aftermarket heavier duty clamps to replace or supplant the weak factory one.
Thrust Bearing wear beyond the wear limit will result in a spun bearing and often machining of the girdle and ultimately cracks. If the bearing has spun, odds are you need a new short block as crank and block/girdle will be damaged. "Visible" symptoms are an engine that turns over slowly with the starter motor, or can't be turned by hand - this means the bearing face is gone and there's block contact - so the engine is already toast. Another common symptom is measuring zero crank end play - although that can be measurer error. If you measure zero end play, pry harder
