I have spent 18 years into solving this EXACT same symptom for thousands of shooters world wide. If you want to understand why and how this handloading problem occurs (and how to solve it) visit this explanation about belted magnum calibers.
http://www.larrywillis.com/answers.html
It describes everything in detail, and a whole lot of well intentioned wives tails will be resolved. Don't feel bad, the topic of reloading belted calibers has confused handloaders for decades.