This is an english translation of a ticket I wrote few days ago.
Have you ever had a lighting where mister Final Gather give you a scene with a whole dirty flicking? The only solution is often to increase render settings and so, render times (when it is still possible ^^') to hide the misery.
And yet, sometimes, increase all settings like a pig is not enough, simply because Final Gather can not handle your case.
A solution to drastically solve (but not always) this problem exists: Importons.