Mark, it really is more than likely is compound and if not temperature, then track conditions. (Car balance may play a role as well) We have seen this sort of behavior from the tires and the cars before. And it always happens when the season changes. It all comes down to choosing the right compound and getting your setup (and driving style) right for that setup.
Build it strong.
Keep it simple.
Make it work.
(Leroy Grumman)