Any exhaust leaks? Check for those (even if you can't really hear it), as it'll pull air in at low speeds until the high-speed flow/pressure is enough to push it out. Will throw O2 readings off - my DP has cracked on me twice and have seen this pretty evidently

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'95 GS w/ t3/t4b
FMIC, diy piping & RFL BOV
MS2-Extra Fuel&Spark
50 lb/hr Holley injectors w/ 1:1 RRFPR
JE 8.8:1 dish w/ Hastings Rings
Eagle Rods w/ ARP bolts
ARP Head studs
Mild port/polish head
3" custom DP/cat section, 3" PIT catback
dual MBC - 12 psi / 20psi