Backfiring?

When you first start the car up it runs seems like it's running good but when you hit the gas it starts popping and sounds like it's backfiring like through the air intake. It has got a bad miss and has no power, but if you aren't on the throttle it seems like it's running fine but as soon as you give it gas it starts popping and backfiring and doesn't have any horsepower. What could cause this? I've changed just about everything on it and I'm stumped. I've never had one act like this before. Please anybody if you've ever had this happen let me know what it is.