Well, faster powders give a different pressure curve, as will using a lighter, shorter bullet. Have you tried the faster powders with the heavier bullets? I'm not a fan at all of the "obturation" method of fitting a bullet, (smash it into putty in the hopes it will fit!), but sometimes it works.
I'm in the group wondering about sizing damage in the seating/crimping process. I may have missed it, but have you pulled any bullets to check? A pitted barrel is always going to be more of a headache than a nice, smooth barrel. It may be worth a try to coat the pitted sections with something along the lines of liquid alox or other lube. Let if get in the pits and fill them and then run a patch through to "iron" it into the pit. It's worked for me on some real rough surplus barrels a couple times, but it's no guarantee. You are in effect "seasoning" the barrel manually. If you've had to scrub the barrel after every session you may need to just get it shooting and never, eve touch the inside again- or change loads either! I have had at least 2 rifles I recall that responded to that method.
I think it might be worth your time to take your alloy and add 2-3% each of Sn/Sb, preferably from something of a known value with some of the vital trace elements like arsenic in it. Good old fashioned WW would be my choice if you can get some old stuff. A pound or two of juiced alloy would tell you what needs knowing- if it's the Sn/Sb thats needed. Usually if a mix has an appreciable amount of Zinc in it you will see evidence in casting, like it won't cast for crap! The possibility that you have a super Tin rich alloy exists. You can sort of make a wild hair guess at this by comparing the weight of one of your bullets with that of a known example in #2. Any Lyman design should work for this. The factory listed nominal weight is in #2 alloy. I can't recall the formula at all, but basically the idea is that the lighter or heavier your test example runs the more or less Tin it's likely to contain. Tin rich will be lighter.
The other possibility was mentioned in another post- a choked barrel. IME, it's a rare thing, but it can happen. The combination of a pitted barrel and a choked barrel? Trade the sucker off or get a replacement barrel. That or stick to jacketed if you can find a decent load.