Download ClockBlocker - a GPU power-management control software designed for AMD graphics cards. It started as a tool entirely designed for the Radeon R9 Fury series, yet many people have had success with the tool on a whole slew of different graphics hardware. While there may be other tools that already work on your hardware to stop downclocking, this is potentially another option available to you.
Specifically, it was designed to do so on cards operating with the newest power-management scheme -- where customization of AMD's CCC profiles no longer works to lock clock speeds at the maximum. What makes the tool unique is that it does this in a very generic way through creating a near zero-load OpenCL compute workload ... this workload tricks the drivers (so to speak) in to believing that high clocks are needed, and thus prevents throttling of the card. Such a method works on many videocards and is not just limited to AMD's R9 Fury series, despite that these cards (and the increased power-management in Crimson) were the sole motivation for the tool's creation.
--It's similar in concept to running a high end game such as Crysis 3 concurrent with playing a low demanding one (such as most JRPG's), only ClockBlocker has a virtually non-existant impact on graphics performance (it doesn't produce heavy workload while achieving the same effect).
This idea has been expanded on in to a full featured profile-based power-management control, of which I am releasing for free for my fellow gamers.
foreground window detection (blocking downclocking only when a program has focus)
minimalistic UI with emphasis on unobstructive use:
-such as, automatic fullscreen detection as a clocking rule (both for Windowed Fullscreen and true Fullscreen modes)
Multi-GPU ClockBlocking for you guys out there that use Crossfire
automatic "instant" effects of clock profile changes, without restarting applications and without saving them
attempted safe handling of cases where GPU acceleration ends (driver crashes, driver reinstallation, etc)
best-effort config storage w/ a backup copy, for all you overclockers out there that're going to hang your PC's while editing profiles (you know who you are!)
-Settings are also file-based and not stored in registry for easy copy between OS installs and machines.
More info on this software can be found here in our forums where the programmer is active to help you out.
Version 1.2-6.4 & 1.2-6.5 (watchdog tweaks, sanity-checks):
-watchdog interval can now be set.
-watchdog INIT reason is now clarified in log.
-Many safety-checks added before ADL helper takes actions.