Chris: Maybe I read more into Baron's post than I should have. I thought what he was describing was static airspeed instability AND dynamic (oscillatory) power instability in pitch.
IOW, the machine speeds up or slows down hands-off (at constant throttle) as I described AND it tends to porpoise when throttle setting is changed.
Either condition could happen because of HTL and a lack of H-stab. The second also could conceivably happen even with LTL, if there is no H-stab and, perhaps, a rotor with very little damping (i.e a high-RPM, lightweight one such as a Bensen rotor).