i would think that this is the nature of corona. it refines the picture with each sample pass. do you shine light through a glass window? i once had a problem where i turned on cuastics and i had black dots all over the floor where light shined through. use the frame buffer and do a region render. if it cleans up after a good while and you still see those, there is probably something funky in your scene that does it (material/lights). try isolate/delete objects in your scene and re paste them step by step. render only the bare minimum and gradually put in the stuff you need and give it a render everytime. focus on one area witha region render to speed up the process dramatically.
i do that whenever i have no idea whats wrong. this way you know relatively fast whats wrong.