The last thing I'd suspect would be software, virus or any of the hardware changes you've made. Much more likely - and I know you've examined the buttons for freedom of movement - is an issue with the electronics inside the monitor that are causing occasional contact to be made. I'd be surprised if that was visible to the naked eye. The two audio controllers/drivers you had, btw, would be a function of your original, plus the one needed by your graphics card to deliver audio via HDMI, I think. The fact that disabling one temporarily improved things was probably just coincidence. I'd suggest your monitor may be on its way out - especially if, as you say, it's getting on a bit.
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