Sounds like they are overheating. What is the temps of the cards?
both of them at 100% activity and at around 75 celcius.
Today, I wake up and are at around 68 to 70 celcius.
I'm not sure what you're trying to say. My original statement still stands. It sounds exactly like the card is throttling from overheating. Though I do admit that if the temp is actually 70C and not closer to 90C, it's a bit less likely. However, if the card is throttling (and perhaps not coming back from a throttle, though I've never heard of that), that would account for it.
Get a program that records the history of the temp, like MSI Afterburner or, if you're using Linux, just write a quick script to to aticonfig --odgc --adapter=all every 30 seconds or so and append it to a file and see if your temps rise too much at one point.
But seriously, what you describe is exactly what I see on my bank of 5870's if they are overheating, except they come back from throttle, but I am using a custom BIOS on all of them, so I don't know what your BIOS is telling the card to do.
It could be that your BIOS is set to throttle at a much lower temperature than other BIOS. Mine throttles the card at 90C.