Irregular shadow buffer probs

Hi I’m a noob here, not sure where to post this and I don’t really know how to explain the problem I’m seeing. But whenever I’m doing animations and using irregular shadow buffer spotlights, I can get a strange flickering on some frames. I can never predict if and when it will happen but I’m ending up having to junk so much work. The size of the problem can appear to be affected by xparts. Any ideas? Just when I think I’ve cured the problem it pops up elsewhere in another animation or another frame. I’ve been struggling for ages with this and I can’t explain the problem well enough. I can work around it by using something other than irregular shadows but I don’t like the other lamps and the rendering times involved.

Pic 1 is how it should be, Pic 2 has the problem, one half is completely missing shadows. Pic 3 is another example of the problem, rendered in 4 xparts two are missing shadows.

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Are you using aproximate ambient occlusion?

No, just checked through my .blend files, not using it in anything.