ZFHP22 in Classic Range Rover, downshift problem in summer

[93 4.2 LWB RR]

Did some more driving today with the kickdown cable connected, and I think I overstated that the situation got worse (I was probably biased, as I was hoping/expecting it to be smooth after the disconnect). I'd say it is the same, no difference in clonk rating between the two situations. The clonk is definitely not consistant, and only happens at low speeds. Sometimes it shifts smoothly.

At this point, I think we are down to either a problem with the valve box or a stretched chain. It doesn't sound like it is doing damage, but it sounds like it could do damage. At what point should I consider replacing the chain?

Thanks again,
Mike