Hi,
Very odd??....my first though is that if it's not working only at certain times then it's probably a hosting issue!......my next thought is, "what the hell could that be???"
There is no functionality in NB_Store code which stops the add to cart button working, so I think maybe a good idea could be to restart your app pool for the site when this happens and see if it solved the problem, if so then it could be linked to cacheing....exactly what I don;t know!
See if you have any erros in the event log for those times and also check if you get any error on the page when you click the "Add to cart" button.
Dave.