Pontiac Repair: Missing when trying to accelerate.., muffler shops, grand prix gt

Question
99 Grand prix GT,3.8 litre series 2just over 200k runs good at an idol spits and sputters when you put your foot in to the throtle,which then in turn makes the check engine light come on.It's been put on diagnosis came up with random cylinder missfire and Mass Airflow changed plugs and sensor Still spitting and sputtering...Any Ideas???

Answer
Hi
I would also check the plug wires and the coils first.
Then, I would check the Catalytic converter. If it is doing it while accelerating, I would have that checked.
Most muffler shops will offer a free or reduced rate on checking the Cat.
Hope this helps.
Good Luck
Pawl