Two things. One is, if the flaps go down too far and the nose is held high on approach, they can blank the stabs and cause the oscillation. Check the flaps and be sure they don't go further down than the recommended range, and maybe even a bit less. The second is the CG, or course. Mine is actually at the front end of the range and I have to hold a bit (not too much) of down elevator when inverted, but it is totally docile in the pattern, so I leave it that way...

Bob