You are too worried about the temp of the lead!!!!!! If you let it cool,it can take 10 or more minutes to get back to a good pouring temp,hence slushy lead.
You mould has to be up to temp,or very close to it,or the lead will cool as you pour it and thicken.
The recipes you are using are old,back from the days when wheel weights were blended differently?
Do some searches and you will find a lot of info on the net!!!!! It is not hard to cast bullets