Hey Dan, if the power valve was blown, then at idle you would have a rich situation. A blown power valve also would not cause a back fire.

A vacuum leak like GMC said might be the problem so just check for broken hoses or hoses which are not in place.

If your two problems are related, my guess would be that your timing is off by 15 degrees or so and your cap is cracked or you have moisture in it,....I am talking about the cap on your distributor. Yeap, you might be having a little moisture problem in your own cap over this, but that is all I can think of and both of these would be easy fixes.