2003 5.3 - Low RPM Miss

[AnimalMother]

I've got a 2003 5.3 w/ BTR Stage 4 cam swapped into my 71 C10 utilize a Terminator Max wiring harness. The truck has run flawlessly for 6 months, but recently has developed a low RPM stumble that I cannot pinpoint. From idle to about 2,000 RPM it has a rough idle/stumble but as soon as I get above that it smooths out and runs perfectly. I've replaced coils, wires, plugs, injectors, fuel pressure regulator, checked all grounds, fuses, and all connections and nothing seems to change, and the truck smells as if it's running rich. Next up is the O2 sensor, but hoping someone else may have an idea/solution. Appreciate any help you can provide.