i managed to create the same problem on a vintage Dura-Ace. crank got loose while riding, rode it about 10 miles till i got home, fixed it (i thought) then it kept getting loose and creaking on subsequent rides. so, as some have suggested, i thought about getting a new crank...

but i thought, what the hell, i'll wrench it on really tight, and if it gets loose, or creaks again, i'll just repeat until it either bottoms out or the arm splits at the taper. as far as i was concerned i had nothing to lose from trying. guess what? it eventually never got loose again or creaked. i think i did a "DIY cold forging" job on it.