As an FYI, I cast everything with a lead pot running at 670F and below that when new ingot is added. I keep the mold at the point where it is just lightly frosting near the base. When I see that, 1 second on a damp towel pulls enough heat out to prevent it getting too bad. I do cast fast with a 6-cavity Lee mold to keep temps up. Keep the stream running constant when you're filling the mold and that has a lot to do with casts/hour and keeping enough heat in the mold to keep it just at frosting temps. I cool the sprue every third or fourth cast if it seems that it is taking too long to harden. That also keeps the casts/hour up and the mold hot.
As you've learned, mold temps matter.