gonefishin', ,,, I've used a hack-saw many times to cut thru 6" thick lead bars....takes a bit of work though,,,if I could get my hands on those massive 3' long bars again,...I'd try a sawzall with a long blade,,,don't put more lead in the pot than you can comfortably pour...the pot can grow heavy while your pourin'...
Fishman, cut the nipple and file away...it won't take much effort,, just a little more time
I've had symptoms like yours due to the lead being too hot. Could be the quality of the lead too...experiment, you'll figure out your problem. Does it happen right away? Try putting the pot on the ground for 5 minutes....see if it gets any better... I'm with Timnam on this one