loosing power/backfire through carb

I had been experincing a loss in power to the real wheels failry consistently for a few days, as well as a consitent popping back fire through my carb while letting off the throttle. I tried to figure what was wrong and was told from my mechanic friend my distributor was not putting out proper voltage (or something like that). I replaced it and the coil and a lot more power came back. However, I still get an almost inperceptible loss of power when cruising - power comes then goes pretty sporadically and the drop in power is much less than before, but still is there. Also the popping through the carb is still there. I restarded the timing a bit and the back fire dminished but still occurs less frequently.Oh backfire only occurs when in gear moving, not in neutral idling Any ideas??
Oh almost forgot
65 dart gt
273-v8 4barrel holley old style street dominator intake mild comp (20-220-3 )cam, eddy 500cfm carb, electronic ignition/mopar, doug headers, rebuilt with about 40,000 /904trans rebuilt with about 20,000
Any suggestions for finding out what is the prob? I have never had any engine trouble before