ExponentialFogHeightFog, Volumetric Lights and Render Errors

I have a scene with ExponentialHeight Fog (Volumetric Fog enabled) and a few spotlights to cast fog under the light. However, it the fog does not render, basically flashes on and off in renders.
Plus, in the viewport it flashes on and off (dependent on camera distance) Advice?

I had such a problem a long time ago. As far as I remember, two things helped: a larger angle for the spotlights and a longer one … I think it’s warm up time in the sequencer.

Why would I get double renders? ie. 0098, -0098?