[averysadman]

I recently got a 270X, very happy with it but I'm getting some strange results when overclocking. Using GPU-Z I have been noticing that under seemingly random conditions the core will underclock itself, this seems to happen at all settings beyond stock.
I've not been able to determine what causes it, but it's reproducible. My overclock of 1150MHz will drop to 1094.4MHz, at 1140MHz it will drop to 1082MHz, at 1200MHz it will drop to 1142MHz, only at the stock 1070MHz does it maintain a constant speed under all conditions.
Like I said though, this only happens under conditions I have not yet determined.

Lost Planet 2's DX11 benchmark stays constant.
Unigine Heaven has many drops.
Unigine Valley has a small number of drops.
Crysis 2 has infrequent drops.
Battlefield 3 stays constant.
Titanfall stays constant.
Furmark constantly drops and rarely stays at the custom clock speed.

This sounds like a power issue, but it happens regardless of what the voltage is set to and the voltage always maintains a constant value even when the clock speed drops, and if my PSU wasn't able to supply enough power, surely that would be reflected in the voltage graph?

I've tried many different settings but nothing seems to impact this.

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Problem solved!

The issue was related to the power control % value, without raising this value past 0% the card was throttling when drawing too much power from the PSU, independent of the voltage settings.

Raising that value to +20% has solved the problem. 

[averysadman]

Problem solved!

The issue was related to the power control % value, without raising this value past 0% the card was throttling when drawing too much power from the PSU, independent of the voltage settings.

Raising that value to +20% has solved the problem. 

[phillpower2]

Glad you got it sorted averysadman and thanks for posting the solution so that it may help others