Definately a sizing issue. Are you bumping the shoulder at least .002 back? If you are not measuring the shoulder (especially on belted cartridges), then chances are you are not sizing it back enough. I had similar problem with my 300WIN. Chambers fine on an empty FL case until it was shot at the range.

I finally went and measured it and it did NOT bump back the shoulder at all. Went and set it back to .003 and all was well.

So measure a before and after FL sized case. You'll find your problem there.