i have this recurring problem with a cup and cone bb: it keeps coming loose. now, i've opened and re-greased this bb a number of times in the past without problem, but ever since my last re-grease the bb can't stay tight. since then i've had to re-tighten it about once every couple of days -- i tighten it up on the adjustable side until there's no play on either crank arm but then, within 30 or 40km of moderate riding it starts to loosen again.

any ideas?