I've discovered over the years that these kinds of bottlenecks can be caused by the most unpredictable and insignificant things installed on a system. A cure on mine after an upgrade was discovering that one of the bits of software installed it's own bundled anti-virus software which was running in the background, and it was the combination with AVG that I normally use that caused my issues. I'd try a different driver and it would work for a while then suddenly click and pop. I started to uninstall stuff and only found the other virus software when after uninstalling, a message popped up asking if I also wanted to remove the other software!

Glad you sorted it.