Young Chang tuning instability

[brittanykirk272 at comcast.net]

I have regularly tuned a Young Chang PF-250 for about a year, and it sounds horrible every time I come back for another tune.  In the tenor section, one note might be +15 cents, while the one next to it is -15.   Unisons are completely unacceptable.  Bass section and high treble seem to be okay.

The person who plays this piano is a "Horowitz", plays very hard, but I still think the stability could be better.  Plate bolts are tight, tuning pins have sufficient torque, I have seated strings, and I pound the heck out of it every time I tune.  

Ideas?  Can not enough downbearing cause tuning instability?  

Ms. Kirk.
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