1998 Pontiac Bonneville SE 3.8L Backfires/ Sputters

1998 Pontiac Bonneville SE 3.8L Non-supercharged engine. When I start the car it starts fine. Idle is good. When I put it in gear it sputters like a Model T. When I hit the gas it bogs down and hesitates. At a stop light if I floor it it backfires from the intake. Then sputters for a second then takes off quickly. Between 45-55 mph it sputters and bogs down. I'm not sure if it's fuel, fire or back pressure.
Fuel system? Ignition wires and plugs? Timing? Catalytic converter?
I recently changed the spark plugs and about 2-3 weeks later this issue started.
Idle is fine only when i hit the gas do i have problems. I can drive down the interstate at cruising speeds mostly with no problems. Take offs and quick starts are the only time i have problems.