Temps would generaly be the reason it throttles back but I can see that your temps were fine. The power usage will only pull what is needed. I can set 300% on my cards but that does not mean it will use 300% more power. You need to monitor the GPU usage and not the power. If this is dropping down, something isn't correct and this is normaly sorted with different drivers.
Ok thanks,
My gpu usage always stays on 100%
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I been chatting to a guy called CrazyNutz who is working on a new bios editor which might be able to stop it throttling
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I'll let you know how it goes and post up some benchmarks when I get my new rig.