Actually, I remember terror alerts being issued many times when nothing major was happening (state of the union address, elections, etc.). While this isn't proof that the government isn't messing around with us, it certainly questions the integrity of your theory. It's another one of those 'damned if you do, damned if you don't' things. If they don't alert people and there's a terrorist attack, they're a bad government. If they do issue a terror attack alert and there isn't one, they're 'messing with us and keeping us afraid'.