I think the crappy aluminium bit that the screw tightens onto wears down, so eventually there's little or no flat surface for it to "bite" into - this means a small jolt and it's loose again. Anyhoo, spring sets for spyders cost next to nowt, so use a weaker spring, and dial it all the way in. That way, even if it does move, it can only go down... and at least you're safe. Won't help much on tourney rules though...