If < expression

Hi looking for a bit of help on an expression, I currently have a displacement map controlled by a slider control, I'm using this expression:

posterizeTime (4);
w = wiggle(8,40)
Math.max(w,0)

To make it wiggle upto 40 pixels, what I want is for it only to displace when between say, 20-40 (I still want the wiggle to range from 0-40 - so that it happens less often - I just dont want it displacing by like 1px etc).

So basically I'm trying to get a stronger effect happen every so often, rather than cycling through

Probably very simple just don't know my way around them too well!

Thanks