Hi Steven,
Are you having this problem on last Sunday?? what springs you are using at the moment? I found this occur when the tires are over-heating and you experience this because it's already afternoon during your 3rd round and the ground temp is about 5X degree.
Next time try to run soften springs all around and that will lower the chance of over-heating the tires and gain back the grip. Also don't use "too much" additive especially when the tires getting old, maybe just a very thin layer of WD-40 and rip if off within 5mins is good enough to clean the tires.
I hope this will help you
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