Ive got a similar problem in one of my cars. My problem is with the distributor. I'm not getting the appropriate spark advance, thus causing a backfire and a horrendous bog at hte lower rpms. This all started when I replaced my distributor with a used HEI. I am thinking that it is worn advance weights or springs on the distributor. It could also be a worn distributor shaft (too much "play" side to side). I havent pinpointed it yet....but I would do some testing of your distributor.
Get your timing lite out, and check your timing at different rpm points. Have a friend rev and hold the engine at a constant rpm (like idle, 1000, 1500, 2000 etc) while yuou check the timing. this will see how your spark adavance mechanism is working in the distributor. While your checking, look for spark scatter, or check to see that the distributor can hold the spark steady at each rpm test point. If it cant, and rpm is being held constant, you could have a worn out distributor shaft or another problem with the distributor.
Can u give us some details on your distributor?