I've had this exact thing happen to me a couple of times in the past. And I can sympathize with you, about how absolutely maddening it is! But I eventually figured it out, so I'm sure you can too.
You checked all the obvious things I would have recommended, so all I can do is tell you my experiences.
I eventually discovered that I had started doing a couple of things wrong, that added up. For me, one of the big things was I started accelerating on curves before I got to the apex. I have no idea why I started doing this, but when I realized it, and went back to making sure I had at least hit the apex before hitting the gas, it made a big difference.
Additionally, I had started braking later than I should have. So basically, I stomped on the brakes before the corner, then stomped on the gas, and never gave the car a chance to balance out.
Even if you're not doing those specific things, you've obviously changed something. You might consider driving some laps trying to be smooth, without even looking at lap times. For me, when I try to bear down and go faster, it inevitably leads to me going slower.
Good luck - you'll figure it out.
PS: Oh yeah, one other thing that helped was trying cars I absolutely did not like. After driving them for a handful of laps, going back to my normal car(s) was like swinging a weighted bat, then swinging a normal bat.