1990 350 TBI questions

[SoCalSlaughter]

I have a 1990 Suburban with a 350 TBI (Lacking in power department) Im looking for ways to get some more power out of her. The truck can already out run my girlfriends 1997 Silverado with 350 on stock rims and tires (Im on 35's) But when wheeling I can tell she needs more power. Ive done a few bolt ons, dont have headers yet though, kinda costly for CA smog legal headers. Id read up on chips for the ECU, my transmission has been rebuilt with a RV shift kit, NP241C transfer case. I have noticed the CEL come on a few times while driving, after getting code its the O2 sensor code. Comes on but turns off while driving. ALSO! haha I put an edlebrock performer intake manifold on the motor, but recenlty have noticed a "puff" or small back fire sound from the truck on idle. Its very subtle but makes me think intake leak or burnt exhaust valve. Any advice, knowledge or hints would be awesome! Thanks!!