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I would say 4 times out of 5, when weird stuff starts to happen to a computer, and something is overclocked, it's because of an overclock. Even if that hardware used to run just fine with that same overclock for years: overclocks are not a static "set and forget" thing. They will change over time as the sillicon physically degrades (both due to natural aging and due to the additional stress from the overclock). You changed out a power supply, so the voltage regulators on the motherboard may behave differently with the new power supply, causing your old overclock to suddenly be unstable. It could be your new video card stresses the power differently, which is interacting. There are dozens of different variables, but the common denominator is the overclock.
Turn everything to stock - CPU, RAM, and GPU, see if it still happens. If the problem goes away, you just need to find a new safe overclock level; your old one isn't good any longer.