I cast .45-70 500grains in a Lee mold. I know I get best results when lead and mould are hot. Just below frosting temperature. The mould gets too hot fast, so I cool it down on some wet rags or sponge often during casting.
I was wondering about the time it takes for the sprue to cool? For me its 3 to 5 seconds when things are going well. Was your 35 seconds a typo or does it really take that long to cool? If so, I think your mold has gotten way too hot.