Slow_96_formula

Hey guys, first post. I've been around these forums for years but finally found a problem I couldn't fix with searching, I have a 96 firebird formula lt1, no extreme bolt ons,just exhaust.

Problem is I just got my pace setter long tubes installed the car ran fine prior to the install, I'm running a slp lm1 exhaust with an ORY, the car ran beautiful for about 4 days and then started stumbling, I scanned it and came back with p0300 (useful) and a p0400, I've checked the vacuum lines to the egr but haven't taken it off to see if it holds vacuum yet, do you guys have an tips? I will post here if I find a solution anyway, but cheers!

Slow_96_formula

Update! After some driving with my scanner I was lucky enough to pull a p0305 out of her so I'm gonna be checking the cylinder 5 sparkplug and wire, I haven't looked since my headers were installed, I took it to a shop(don't shun me it's the first time I've ever taken her to a shop) but ima go ahead and have a look tomorrow when the shines out. I'll update you guys again tomorrow, oh also codes popped for the o2 censors which is to be expected with the cat delete, that'll be tuned out later bUT for now, the misfire