It's the barrel. Some factory barrels shoot, most don't. The barrel is changing when heated, whether it is unrelieved stress, inconsistent bore dimensions, or some dynamic you'll never identify, you can't fix it, there is a reason good barrels cost twice as much as factory barrels. Bite the bullet and put on a premium barrel and never look back. In the long run a barrel is a consumable, just like bullets, brass and powder. Your time and peace of mind are worth something, I have done all the things you are considering. This advice is worth what you paid for it, but it's true.