I've made an animated scene with 2,000 frames, which rendered fine but then I worked a bit harder on later frames and for some reason it now has shadow artefacts when rendering later frames! Help, any ideas please?! Stills look fine but animated you can see the shadows strobing inconsistently (see gif below).

via GIPHY

Link is here if that gif doesn't work

Lighting is consistent (not animated), what would cause these artefacts to appear? Can't understand it. I'm using Poser Pro 2012.