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If you are using the Hardware Monitor, then I have found that if you apply an OC setting and just let it idle, then it will drop Core and Shader clocks but then revert to "right "clocks when you load up the GPU.
Bear in mind your overclock is only likely to apply to performance 3D so it will drop to whatever you have set it to for 2D until such time as the GPU is getting worked by an intensive 3D application (eg. game).
Run a game then go back into Hardware Monitor (make sure you've hit "record") and see what the clock speed were doing whilst you were gaming. It should be as per your OC.
use atitool (scan for artifacts) to make the card use the 3D settings. it's also a nice easy way to test your o/c stability while playing with new speeds.

