I'm not sure about the PPC family but the 68K family should not be able to do this... on a 86x it's theoreticly possible as some of those CPU's have a downclock feature when they are experiencing overheating so if the CPU is damaged in a way that over time makes it more and more prone to overheat during normal usage or even idling then I think this would be a theoreticly possible phenomina.
