Thanks for your answers and for shearing your experience.
Steve, wow that’s an impressive background!! I remember Cast Performance Bullets.

The fire is on, so I will be casting in some minutes.
I will try to cast at 750-775 º and I will not cool the mold every time.
I get the idea of cooling sprue plate from castboolits.com. It looks good and it let me cast without interruptions until I have no more lead in the pot.
The mold never get too hot. After the cavities are filled, I put the sprue plate over a wet towel for 1-2 seconds.

I will let you know how it works.
Thanks
Martin