[netpog]

Since you think it's likely to be that diffuser, and NOT so likely to be thermostat et al, then I'll order a new one. Maybe easier to replace it than diagnose it, at this rate.

And I think I ought to buy it from appliancepartspros.com, don't you?
[UPDATE: I just ordered the diffuser. And a Whirlpool fridge repair manual, just for good luck.]

That service manual is very cool! Thanks for that link. (I don't have the electronic-control version, but the diffuser replacement is the same.)

That control-box-removal thread you sent me doesn't apply to me. That said, I won't NEED to remove that box, if the problem's in the diffuser unit itself.

Thanks much!