Backfire whilst shifting up a gear above 3.5k rpm.

Hey yall new owner here. I've been driving my speed3 lately after changing my spark plugs Prior my car was running terrible and backfiring constantly in every gear even at low rpms. Code p0303 had popped up.

I changed the spark plugs and that fixed the issue code went away. 300 miles later the car is running amazing probably a placebo affect but it pulls a bit harder now(previous spark plugs were really burnt).

Only problem I have now is sometimes when I'm up shifting at around 3.5k+ rpm the car will give off a big ole backfire. I got pulled over, overtaking a cop because of it yesterday.

I ordered ngk laser iridium gapped at .27 . Running 91 autotech hpfp, cs injector seals,cs catback, cs power series 3" intake, cs dual vta bpv and cs tmic. Thx for any help im thinking im likely just gonna need to get another tune soo dial the car in.