Backfiring at certain RPM on rev decline???

Well i've noticed this started during the last hot laps of an auto-x event, and it is much more prominent and frequent when I'm open header. Say for instance i'm in gear (doesn't matter which, just any) i can floor it to redline without any hiccups (sounds and pulls nicely), BUT if i get off it completely and let RPM's decelerate on their own right around 3K rpm i back fire. So I'm stuck trying to guess what kind of threshold i'm crossing. I think my MSD ditributor timing might have something to do with it, as i have it set to hit full advance at 3K rpm. But I am unsure as to why it only occurs on deceleration and no acceleration (since there's more of a load during accel.). So I opened and cleaned my carb. and pulled the vacuum advance line from the carb to the distributor and it still does it.. ...I wonder if perhaps the motor is experiencing a lean moment?? If anyone's followed what i've done recently (new afr's and COMPLETE msd system), maybe they can guide me towards checking other key dynamics I am not observing. OH FYI: Directly after my headers, there is an H-pipe connection which i thought would equal out the exhaust pulses and eliminate possible after-fires.....guess not.

Any help would be greatly appreciated!