Regarding overclocking the core, when I got my HD 7950 some years ago and I hit 1125 on the GPU and the max memory of 1625 I noticed the same fluctuations. I then ran msi afterburner (which also extends clock limits, but not power limit) to get a nice graph of what was going on, turns out it was throttleing up and down between 925 and 1000 dispite not going over 79°.. So i figured it was hitting the 20% power limit :/ it took me some time to find how to pass the power limit, there turns out that there's a registery edit that can be made to allow up to 50% power limit. I set the limit to 30% and I believe they hide this because they don't want people cutting down the life span of the cards by going to past of what the MOSFET's and capacitors are comfortable with.
Now I have the card very happy at 1100 GPU and 1600 VRAM, I always knock it back a bit so it doesn't show its self unstable later on. Because even with something like furmark, it won't always show up unstable until you get into some games or benchmarks.