Placebo effect in action 
Every time I replace a component, I think the engine is smoother until it is clear that it isn’t. The only cure for that annoying idle stumble I’ve found is to run it in open loop (lambda feedback disabled).
Then, as Kirby correctly pointed, the mixture is richer and the engine speed is silky smooth.
O2-sensors always cause rich/lean oscillations and the idle speed suffers. As suggested in some of my earlier posts ([xj-s] 6.0L V12 idle woes -- it never ends - #7 by CJ95) the Lucas ECU is not fast enough to adjust.