Weird core frequency behaviour

[techfede]

Hello everybody, I'd like to ask you all for some help with my system.
I've noticed lately that my xfx hd 7950 (900 MHz stock clock) sometimes drops to 850 MHz throughout an entire "3D session", that is for as long as I play or use benchmarks.
I'm using mostly Unigine Heaven because I want to overclock, but often the clock gets stuck at 850 MHz no matter what I choose it to be in MSI Afterburner, and yes that includes also bringing the clock under 850 MHz. For some reason it just doesn't want to change.

Just to be clear, it doesn't happen always. I've finished lots of benchs in Heaven with some OC on the clock, but I've never been able to do so in firestrike, where I'd always get stuck at 850 MHz even with stock settings.

The only software I'm using is MSI AB and the latest Radeon Software version.

Let me also say that my motherboard is probably defective, it's an old 1156 motherboard that would cost a lot to replace, so I'm just waiting to make an entire new pc, and because of that I really can't be sure if the issue is software or hardware.

I hope somebody will be able to help or to redirect me to a possible solution, so far I've been unsuccessfull.
So, thanks in advance, and if there's something I didn't say or you need more information, don't esitate to ask

 

[maddogfargo]

vSync off?
Load temperatures?
Fan speed?
Custom fan curve?

GPUs will throttle as they near thermal limits. This usually only happens when running at 100% for long periods.

 

[techfede]

vSync off?
Load temperatures?
Fan speed?
Custom fan curve?

GPUs will throttle as they near thermal limits. This usually only happens when running at 100% for long periods.

Right, I forgot about temperatures!
I'm using a custom fan curve in Afterburner (I try to keep the fans spinning as low as possible because on this model they are very noisy, at 45% - 50% they sound already like a vacuum cleaner).
Temperatures have always been below 80°C (the actual highest reported temperature was 76°C after some loops in Heaven), but what doesn't make me think it's termal throttling is that the change in core clock occours as soon as I start any 3d application (again, when the issue actually happens), not after some time.

It's like now I'll run Heaven for a while, temperature stabilizes around 75° with the correct clock and everything, then after a while I'll start it again (after doing nothing for a while) and as soon as I launch it the clock goes to 850 and won't move from there.

Edit: I'm not using Vsync in Heaven, I usually do in games if I can and if it's needed (I actually tried multiple times to force it on The Witcher EE but failed each time). What would vsync change?

I was also thinking that, being the gpu used, the precedent owner might have flashed the bios to a custom one, thus getting this issue.
How can I check if I got a regular bios? GPU-Z?