Electro-migration occurs over time, leading to CPUs that were previously stable at a given clock / voltage combination becoming unstable...after long enough they even become unstable at stock...but it generally takes a long ass time for that to happen. The VRMs on the board also deteriorate with load, faster if they're getting hot...eventually leading to unstable VCore which can result in...instability. Further the PSU degrades over time, eventually resulting in ripple, which the VRMs have to work harder to filter out, leading to them deteriorating.
In short...yes. However it doesn't mean all is lost in terms of overclocking. You might just have to dial back a bit from where you previously were, lowering the core clock or raising the voltage a hair. Really depends on what the source of the instability was.