Learn English – A pair of words that mean fast- and slow- changing

I'm looking for a pair of adjectives which mean, respectively:

• Fast/Frequently-changing
• Slow/Rarely-changing

As an example usage:

Due to its everyday use, the balance of a current account can be quite fast-changing, but that of a savings account is more likely to be slow-changing.

And as a demonstrative graph: the value denoted by the red lines is more fast-changing, whereas the green value is a lot more slow-changing. The size of the changes is irrelevant – only the frequency is of importance.

The best I've come up with is volatile for the fast-changing part (probably closest to the third definition at dictionary.com), but I can't think of a suitable antonym. To me, stable has more of a connotation of not changing very much when it does change, as opposed to not changing very often.

By contrast:

If the green and red graphs above are frequently and infrequently changing respectively, then the orange, blue and purple graphs are changing continuously (infinitely frequently) – they're all way off to the "frequently" end of the spectrum.