I found this in a white paper on WIG craft - it seems to go along with our thoughts:

[link=http://www.dsto.defence.gov.au/publications/2058/DSTO-GD-0201.pdf]Australia Department of Defense[/link]

"2.5.2 Pitch Stability
Pitch stability is a measure of the response of the craft to changes in pitch. With a disturbance in pitch the response of the craft can be either stable or unstable. Unstable behaviour results in increasing pitch amplitudes, while stable behaviour results in the craft returning to a pitch angle.
The control of WIG craft pitch stability has been one of the larger hurdles in WIG craft
development. The problem is due to a change in the pitch stability with height. The result is the necessity for a large amount of control power to maintain trim. Early designs and
theoretical studies have shown that the greatest problem is damping the long period (phugoid) oscillations. DSTO-GD-020114
Designs have often shown stability in some regions above the surface and instability in other regions. Pitch instability causes ride discomfort and is a possible hazard to the craft. As the pitch stability is linked to the vertical height stability, large excursions could cause contact with the surface, resulting in high structural loads or failure.
It is now considered [3] that this problem can be overcome using modern control methods and the current understanding of WIG craft aerodynamics. The challenge in the design phase is to provide sufficient control power for stability to be maintained throughout the vertical height envelope."