It's been a while but let me give it a try.
Given:
Higher altitude same IAS. For the IAS to remain constant with increasing altitude means a higher TAS.
Same IAS, higher TAS.
Aerodynamic dampening is a function of TAS.
Less tail angle of attack means less dampening.
Static stability is the "willingness" to change direction.
With the center of pressure forward of the CG as it should be static stability increases with TAS.
Dynamic stability is the "willingness" to return to the starting point and how.
Is it dampening or overshooting the neutral point.
Since the dampening effect is reduced the dynamic stability is reduced.
Static stability increases, dynamic stability decreases.
I had some help figuring this out:
High-Altitude Handling
http://www.flightlab.net/Flightlab.n...u%232BA152.pdf Interesting PowerPoint about high altitude stability:
http://flightsafety.org/files/Append...Operations.pps