Check your ignition coils. Last time I had the same symptoms and it was a faulty ignition coil starting the misfire. The car sounded like it was a gas rc car (idk best example I know) and held back tremendously. When I restarted the car it was fine then out of the blue the sputtering again. Make sure you check the ground straps and the coil connectors are fullt connected. I had one loose another time. Its easy to forget especially ifyou were doing work related to that area. Also my idle was rough.
And btw I like the M bumper. Your car came a long way I remember when you first signed up for the fest and made a welcome thread lol