The silentbob school of getting other people to do your homework for you
As you've pointed out it's largely down to reduced density and viscosity of the air at higher altitudes.
Try typing 'coffin corner' into a search engine. I think this a condition where the Vne coincides with the stall speed of the airframe although I don't have a good definition to hand. (This is of course an extreme case)
Good luck with your assignment.
PS this thread would probably have been better off in the flight test forum
