I don't have a theory about your pressure problems, but I do have a similar experience. Nearly twenty years ago, a good friend built a 35 Whelen on a Mauser action. The rifle gave good performance with handloads, but Mike was not a reloader. When he tried Federal Premium ammo, he got the same results you did. Unfortunately, he didn't follow through with Federal to determine what the problem was with their ammo. I can't imagine why Remington ammo would be OK and Federal not unless something is seriously wrong with the latter.

Bill Ferguson