Its been said before and I'll say it again. Inflate the tube partially when putting it in so that it will hold it's shape. It's hard to diagnose if it's the rim or the tire over the internet, without seeing it, but the one obvious problem is that the tube needs to be straight and not crunched up when you are installing it. I work at a B-shop and I have changed more than my share of tubes and tires for a lifetime, and one of the mistakes I made when I started was cramming the tube in. It resulted in many brand new unridden tubes exploding VERY LOUDLY by my ears. Good luck.