NB demands more voltage than before. Why?

A few weeks ago I started getting frequent windows freezes followed by spontaneous reboots when stressing the system. For a while I thought it was a software or HD problem. I ran memtest and the system quickly froze so I bumped the NB voltage up a notch and now I'm stable again. What could cause a NB to start demanding more voltage than it previously needed? Has anyone else had this happen?

Heat. Anytime more heat comes into play, it takes more voltage to keep things stable (sounds counter intuitive, since more voltage, even more heat). But from my experience, when summer comes by, i have to notch up a voltage on certain things, and it makes 1-2C difference but at least everything is stable thereafter