Marvin,
It's the brass. I don't use Remington brass for that very reason, and haven't done so for 30 years. I've had many factory loaded rounds split the necks when I used to buy factory ammo for the brass.
I have a S&W 25-20 that I've loaded for over 25 years and haven't lost a piece of brass yet. I also have a Savage 23B with nowhere near the number of rounds through, but haven't lost any brass with it either.
You might try annealing those you have left until you can replace them. I never bothered annealing Remington brass but it may be worth a try.
David