Backfires through Exhaust

[AZreGuy]

Car has been running fine and today on exit ramp of thruway the car backfired thru exhaust.

I was running about 65mph and let off gas to slow down when it occurred. It then started chugging with a few more backfires while trying to drive. It seemed to idle fine but pushing down the pedal is when the issues continued.

Any thoughts?

75 450SL - within the last few thousand miles new fuel filter, plugs, wires, cap, rotor, timed, oil changed, drained tank and cleaned fuel screen, removed cleaned and freed up air valve to name a few