The problem of flickering lights and shadows is mentioned in the manual.
It is about rendering animations using IDL, but it may relate to simple raytraced lights and shadows as well.

The solution is to set the Number of Threads in the Render Tab of General Preferences to 1 

Maybe one thread is rendering a bucket getting hit by light, while another thread is rendering the same bucket +1 frame, but doesn't know that it just got hit by light...
...or something like that :-D

 

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