sorrys for all this trolling, but I think I've found the reason
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Using Rivatuner's monitoring device I found my fps go down the moment the temperature of the graphic card's core reach 125°C (didn't check long enough on my previous post). This leads to the core's temperature declining. When temperature is lower (about 108°C) fps go up again. btw, I have done nothing with this card except software overclocking. Even when I lower the gpu and memory speed to 200/200, the temperature still rises, although much slower. Will try to run with my side off.