I have a 2008 aluminum Macbook running Snow Leopard, and I have set it to ask for my account's password after 4 hours inactivity. However, it sometimes asks for it much earlier, after just a few minutes of inactivity. This also happens when waking the Mac up from (a short) sleep, but when that happens, the screen usually stays completely dark: I know it's actually asking for my password because if I enter it, the screen "unlocks" and starts working normally; if I press Esc instead, the computer stops again (and works normally if I wake it up again).
This appears to be a random behavior that I cannot reproduce at will. It happens quite often after a stop, but only rarely otherwise (i.e. without sleeping).
Does anybody have a clue about what could be going on?