WordNet sense 1 (not showing abrupt variations):

level

WordNet sense 2 (not subject to change or variation especially in behavior):

steady

WordNet sense 3 (marked by firm determination or resolution; not shakable):

firm, steadfast, stiff, unfaltering, unshakable, unbendable

WordNet sense 4 (firm in purpose or belief; characterized by firmness and determination):

resolute