BBS a330 managed descent problem (or bug ?)

 When I start my descent before the TD the plane starts a gentle descent (-1000/ftm) till it intercepts the profile path.It then  noses down, literally diving -2000 ftm  ending up below the path.  It tries  intercept the path again by going up at more or less 2000ftm  going way above the path.The plane basically flies like a dolphin.I made sure the cwog was at 30%.no joy.I also made sure the flight plan was realistic but nothing seems to help. Vdev is alwasy + o r less 1000.If I'm lucky.

Am I missing something ?

Thanks

V