dip in memory clock... only thing i can think of is fault memory controller/ ram chip on the GPU card .. they can fail, rare - but if you have i recording or VRAM speed decreasing then increasing should help fight for card RMA testing
It's frustrating as hell, I've only seen HWinfo record the drop in Memory Clock Speed once, that doesn't show up every time the FPS plummets. There is a problem somewhere because the same scenes in a game will run fine again once the system has been rebooted. I've gotten some videos of the FPS drops, first to around the 85fps mark, then to around the 55fps mark with nothing standing out.