1) Constant 2) Fleeting 3) Fugitive 4) Lasting but a moment 5) Momentaneous 6) Short 7) Short-lived or fleeting 8) Very brief Found on https://www.crosswordclues.com/clue/momentary
Mo'men·ta·ry adjective [ Latin momentarius . See Moment .] Done in a moment; continuing only a moment; lasting a very short time; as, a momentary pang. « This momentary joy breeds months of pain.» Shak. Found on http://www.encyclo.co.uk/webster/M/90
lasting but a moment; very brief; fleeting: a momentary glimpse. · that might occur at any moment; ever impending: to live in fear of momentary annihilation. · effective or recurring at every moment; constant. Found on https://www.infoplease.com/dictionary/momentary