aircraft bobbing?

[pepto]

so i'm trying out this fantastic F/A-18 mod (found here https://www.dropbox.com/sh/cx3et8f0y1hilbx/e2m49l9tf-) and im having a bit of problem where the plane bobs up and down with even the slightest input and even at high and stable speeds, is it a problem with the trim? becuase it always seems offbalance, and it can't be weight related because it happens with both full and no payload