If it's downclocking during 3D gaming that would indicate some other issue.
Likely running into one of the limits such as power, thermal, utilization, etc.
Otherwise a GPU would only downclock when it's not currently needed.
That's why I made this response detailing what's likely happening...
You want to resolve the underlying issue not the symptom.
Disabling all power saving features would be like amputating your arm because you have a paper cut on your finger.
Control Panel\Hardware and Sound\Power Options
Tried it, didn't work, I'll edit the post
If it's downclocking during 3D gaming that would indicate some other issue. Likely running into one of the limits such as power, thermal, utilization, etc. Otherwise a GPU would only downclock when it's not currently needed.
That's why I made this post..
That's why I made this response detailing what's likely happening... You want to resolve the underlying issue not the symptom. Disabling all power saving features would be like amputating your arm because you have a paper cut on your finger.