ORIGINAL: beepee

Ed,

More to follow from REAL technical experts, but if everything you mentioned is the same, the only thing I can think to look at is the incidence angle between the wing and the horizontal stabilizer. More pitch stability with more negative angle of the stab relative to the wing. Less with 0-0 or positive angle stab to wing. There are a number of sub-variables involved too, but this is the first thing I would look for.

Good luck,
Bedford