A year ago (probably longer) Mouser changed the back-end of their site. Before the change they were almost in lockstep with DigiKey on price. Since the change I've notice some prices have diverged and that Mouser's pricing seems to change more often. My impression was that they were using a few "key" parts as loss-leaders to draw customers away from DigiKey.
What you're describing seems like "customer specific pricing". The next time you notice a big jump in price, use a different browser / computer to recheck the price (ideally a computer with a different internet facing IP address). Or, ask someone on the forum to check the price. If there is a difference then they are picking on you (trying to squeeze a little more money out of you).