GPU load drops to 0%, FPS drops in games

I have a very specific problem that I get in a small number of games.
Seems related to the number of entities that have to be calculated at a time or something.

Games affected:
- Starcraft 2 (only certain maps / game modes)
- Warframe
- Mighty Quest
- Diablo 3 (only in rare instances)
- Planetside 2 (very rarely, only in certain facilities)

Here's what happens, and it's exactly reproducible in all of the first 3 games at least:

Game runs fine normally, sitting at 60 FPS on max settings (high for starcraft).

Then as the number of entities (i'm using this word, might be wrong) not just on screenn but in the game increases, it starts to drop.

- units/structures/shots fired on some SC2 maps
- triggered enemies on MQEL (only triggered ones, they're on the map normally no effect, but e.g. in castles where i pull over 100 at once, it drops badly)
- pickups on the ground in warframe (ammo, energy spheres)

As the number of these things increases, GPU load steadily decreases until it eventually hits 0% with suitably low FPS, as the CPU is now apparently doing everything.

Killing the units / picking up the loot drops restores the GPU load and framerate again.

I've tried running all kind of diagnostics, including running kombustor and all the other stuff and all of that runs fine. Most games run fine. But in a few I have this issue.

System Specs:

CPU:
AMD FX-6100 Black Edition CPU: 6 core, 3.3GHz (auto clocks to 3.6)

RAM:
16GB DDR3

GPU:
AMD HD6950 2GB gpu (powercolor)

Drives:
120GB SSD (system)
1000GB HDD (games installed here)

Mobo:
Asus M5A88-M

OS:
Windows 7, 64 bit

Everything is fully up to date, drivers bios etc.
No idea how to fix it, I've tried everything I could find through hours of googling.

It sounds like it may be an issue inherent to the motherboard as people with similar models have had similar issues and no way to fix without swapping hardware.

Any suggestions here would be welcome.

so basically everytime the mothership arrives, the game drops to 0fps?

hmmm. your system requirement is not a concern here.
CPU fine, more than enough RAM.
I am thinking about your GPU.. since when more things to render it slows.

actually try to 'DOWN CLOCK' the GPU if you can.
sometimes when the clock is too high, it starts to stall with increased workload.
This is evident in ALL GPUs, though more evident in AMD over Nvidia, cause AMD lets you MAX the voltage to no limit, which is actually not good.

try to lower GPU voltage, see does it stabilize it a little.
Rule of thumb of a gamer, If overclocking gives you no better performance, DOWN the base clock of the GPU so it is more stable in competition.

I have been running on stock clocks, though I have tried both over and underclocking for diagnostic purposes; it makes no real difference to the issue. It's not a rendering problem either - the games where it happens, especially warframe, starcraft and MQEL, when playing normally the GPU load will set between 35-55%, it doesn't go higher than that.
Other games (with better optimisation, maybe?) which use higher GPU load % when running normally, like 70-99% don't have these drops.

It's as if the GPU is only half committed and when it gets tougher it just gives up.
Is there some way of forcing the system to use the GPU?

Especially in the case of MQEL it's very wierd:
I can have the same 100+ enemies on screen with absolutely no FPS drops, so long as they are idle, i.e. I have not walked into their agro radius. They will stand there idle, all running random idle animations, and it won't cause any problems, still full 60 fps. As soon as I agro them so they start moving, even if I then run off and they're off screen, the problem happens. Actually with all of the games the stuff doesn't need to be in view, just in the level. Which leads me to believe it's more of a CPU calculation thing telling the GPU to bugger off, or something.

For starcraft, normal games are fine, but in the arcade it happens on certain maps especially those with lots of projectiles, e.g. warships map, where you have ships with dozens of turrets all shooting each other a lot. Other maps like zombie ones with 500+ things moving on screen don't cause slowdowns. Which again seems more of a calculation than a rendering issue. Kinda lost here.