Had a similar problem after a tune up. Not as bad as yours sounds, but would backfire under a load. Idle was fine. Re-checked the 'new' plugs, and one was cracked. It got worse as we continued to run, I guess the crack got worse as it heated up. As mentioned, plugs/wires/timing sound like a logical starting spot.
2008 X-14 MCX
1993 Prostar 205