engine sputtering under load

[guzabri]

so i just recently got my GLHS together with a new clutch, headgasket, timing belt, 3 inch side exit exhaust, and tranny. So far ive got like 250 miles on it. The problem showed up today. ive been driving it pretty nice so far but today i felt like holding it to the floor. I did a second gear pull and it revs to like 4500 and then it seems like it sputters and slowly gets more rpms with a little backfire i think. What could be causing this. I tried it in 3rd and the same thing. When i tried it in first it reved to about 5000 then a little bang came out of the exhaust so i let off. The only codes i have are 12, 31, and 55. I had a code 13 and the only way i got it to go away was replace the MAP sensor so that is new. The car also had a new fuel filter plugged in. Is not enough fuel getting to the motor when i floor it and it builds quite a bit of boost. I have the stage 2 MP computer in it. It runs pretty nice just driving around nice. I bought the car last september or so and i dont know how long it sat before that because the guy i bought it from was in rochester, mn and he bought it from pennslyvania. The guy i bought it from only had it for a couple of months because it was too much work for him. Could my timing be off? or is it most likely fuel related? Thanks for any and all input.

[sittn86]

Your injectors could be plugged. It could be spark related, too. Check all the connections in the ignition system & clean thoroughly, relube with dielectric grease.