Had similar issues for many years, also with a GTX 660 TI gpu. Finally decided to update to a GTX 1050 TI and problem solved at last! So yes, I think it can be due to a failing GPU. I think it is a...

I know this is a fairly old thread but I have EXACTLY the same issue as the OP, with same GPU (although I do not have a built-in GPU). You have analysed its behaviour and described it to perfection!...