Yes, you can find that at any hardware store, its made exactly for these type of things, where you want the screw to not move unless you want it to, but still be easily adjustable.
Don't put glue in the core..put it at the tip of screws and let it dry...it's just to make the screws sort of slightly thicker so they dont unscrew themselves..
I've already determined that its a slightly crooked screw which is sufficiently rubbing up against the plastic in the center cap causing it to unscrew, im going to now edit my original post saying that I know what the problem is.
By the way... where would I buy just a core? Is that possible? I think its kind pointless to purchase a whole new cube just for a core because then I could just that new cube instead. So where would I get just a core? I need one for another cube.
Argh i have the same problem too. and im going to a competition in a few days. i already bought some new longer screws but it got even worse!(?) but what the hell is loctite?