Are you sure of your lead supply? Sounds like you might have dirty lead - zinc maybe.
If you are using wheel weights that might be your problem. Trying adding wax (small amount) - be careful because it can flash.
If your temp is too high you will have a blueish haze on top of your melted lead.
I crank my pot up until the lead starts melting and then turn back about half. All Pots are not the same. Think the older pots can get hotter.