Non-equidistant wavy walls

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.

I would check e steps, you might be over extruding a little. Also adjust your height a bit better, it looks you squishing quite a few of lower layers.
I too do not think its z wobble.

It's not e-steps, it's also not z-steps since that would result in a fixed effect in every # layers.

It looks like z-wobble! Since the spacing of the bands gets wider each time, it looks as if the problem should be found in the z-coupling. Is it completely centrically aligned with the stepper motor axis and the threaded rod?