Hey Hilander
I've had the same problem. Some fixes come to mind.
Switch to a softer alloy of lead - most storeboughts are about 16bn
Better Lube in the lube grooves - aka Felix lube, LBT Blue, NRA
Hone the throats out a little to .452 if you are using .452 bullets
Do all of the above and my guess is it will solve your problem - it did for me.
I have also had luck with the Liquid alox as well. Try that first. Liquid alox worked for me as well and might be a good solution for if you dont cast your own. It is a little messy but it works.
My guess is too that you are getting a little leading deposit inside your chamber just before the chamber throat. Honing the throats might help with that.