Dual volume levels

Hi,
Someone will know exactly how to solve this!
In a couple of my VST plug in midi tracks (Omnisphere in fact), in the volume controller lane I have TWO levels: the normal midi controller volume PLUS a weird line which has a mind of its own! Where has this weird line come from? How do I get rid of it? The volume of the track currently acts in an unpredictable way.
Cheers,
Neil