Any chance you got an after market ignition module? If so did you get a warranty? Just saying when ever my car has started to back fire under throttle it's the igniton module going bad. Another thing to check would be spark plug wires, make sure you don't have one burnt through. There's a couple that get pretty hot and can cook if you're not careful.
Just another thing I've had in the past, we re routed the ones that get a bit warm. Good luck and let us know if you figure it out.