Hi Cory, thanks for your reply.
I seem to be getting different artifacts every time I render, and different for every frame.
Here is a recent zoomed in image:

I’m using greyscalegorillas HDRI Sky with global illumination full animation mode, but I have tried all of them and the only one that seem to work is the Sky sampler. I only have a simple yellow material with specular, illumination and colour on, nothing else.
I looked at your other post and have tried to swap out the different elements, new project etc but it all turn out the same.
May be it has something to do with my objects? I have a cloner object with heart shaped splines in an extrusion nurb, and with rigid body applied to the cloner object. They get throw from one spline to the other with spline effectors so the hearts make up different shapes. (Based on the Greyscalegorilla Cherry 7-up tutorial.)
A compromise I might go with is the Skysampler with the samples set really high, like 1000. It looks ok, but not as nice as the full animation mode when I render a still image.
Any thoughts?
Cheers,
-Ole