Non-equidistant wavy walls (no replies)

I am having a problem that I've been having a while with inconsistent walls. There hasn't really been an obvious pattern. But I've just noticed one of my test cubes has shown up a wavy pattern which begins quite close together at the bottom but getting wider apart the further up you get. Because it is not a consistent distance I am guessing this is not z-wobble.

Does anyone have any idea what could cause this pattern of waves getting further apart higher up?

Thanks.