I've got 12,000 miles on my stock 18" P-Zero Neros and I have noticed more squirliness in the past month or two. I have an automatic and I drive with all the electronic nannies turned on, but I can certainly tell that I'm getting a little more wheelspin and a bit more of the back end breaking ever so slightly loose if I get on it harder. I would say that part of what you are seeing is tire wear starting to kick in as it gets to some of the harder compound and the temps start to cool down a little.
1973 Mach 1 - Medium Aqua, 351c, functional Ram Air
2013 V6 Premium - Deep Impact Blue, MCA, 3:31 gears.