Not sure if this relates to your issue, but I can appreciate the frustration/confusion. I note that you actually have two nav-bars, "Navbar Scroll" which is hidden in all viewports, but when set to block, overlays and obscures your primary active Navbar. Just saying, this may have some unforeseen effect when moving down through the site?
I have also experienced this issue recently, where major banner images suddenly appeared low-res (Blurry) for no reason or explainable, implemented change. I have found that toggling the "Image is HiDPI" box in the settings panel (or in the Background image module within the styles panel) helps resolve this.
Hope the extra eyes lead to something - apologies if it's misguided thinking.