Lights playing up.

Hello All,  Been trying for ages to get the lighting right for a particular scene but no way no how. As soon as I add any extra light to the scene [ Spot, Area, Point, or Directional  they just go haywire and cause havoc with the Global illumination AND  the Hdri.  For example,  if I go to rotate the Global light by a tiny amount as long as there are no other extra lights added then that is fine BUT  if there are any of the lights mentioned above present then it is far too unpredictable. even a tiny rotation or a slight increase or decrease in intensity and the effects are massive and totally unpredictable.  I realise that Stager is early in its development but as it is just now  Iray from within Substance painter is superior. [ even though it lacks the flexability to add extra lights ect. ]