Just a though, I'm just learning some of this stuff...
O2 sensor needs to be hot to create a signal, perhaps it's reading lean when cold so adds the +25% then as it warms up and gets a proper signal it takes it away, similar when you rev it to 2000rpm?
Is the O2 sensor fluctuating on idle / when it happens? perhaps check the power to the O2 sensor? is it a factory manifold with the sensor up near the engine or is it further down?
