Glad to hear that the problem has stopped, but it is strange you had to downlclock it if it wasn't overclocked. Maybe it was just running too warm at normal clock speed, that's certainly possible. You might try adding some more cooling and see if setting the clock speed back to normal still results in the problem or not. Hopefully, it won't.
Power is a more likely issue than heat. CPUs draw more current at higher clock speeds; the most likely cause of this sort of thing is that the PSU couldn't supply a stable voltage, either because it's overloaded, or just broken. People with huge modern video cards and stock PSUs often end up overloading them, because the video card draws way too much current.