When I enter the rendered shading mode in Cycles the preview render often gets stuck in an infinite loop: it renders a few samples, sometimes only 1, and then restarts from the beginning. I can't find any logic behind which objects or layers need to be visible for this to occur and no settings have changed afaik. This is a recent problem at work, happening in different files on two different computers.
Any ideas what might cause this?
(PS: Since these are work files I'm not allowed to share them here.)
Extra info edit: Blender 2.79. Standard interiors of about 1.5 - 2.5 million faces spread over a handful of layers, nothing we haven't done before.