How to determine the "variability" in a frequency range?
Daniel, 

Upon re-reading your question, it appears that what I have learned to be known as the 'Gabor Bandwidth" might be useful to you in this case, for you trying to measure 'spectral variability'. (Dilip provided a good answer to my question on Spectral Moments here). 

When I studied it further, the Gabor-Bandwidth seems to really just be a measure of how 'spread' the spectrum is from its mean. (Hence the manipulation of moments). 

Take a look and see what you think.