Assuming my wife gives me some time this weekend
I'll start checking.
I have been assuming it was fuel related since it did get better with the cleaner additions. The fuel filter is only about 10K miles old (like that means anything) - but since it actually runs better under WOT, which would mean highest flow rate, I'm doubting the filter.
I wonder about a bad sparkplugs/coil, again since it gets better under WOT - but then I always learn new things - can they behave in this way? Plugs are Bosch-4, about 25K miles old.
Idle is fine, more or less OK at WOT, poor at mid-throttle. Note, this is throttle induced and is not speed sensitive, I can get the shuddering/misfire if at 15 or 50 mph. Also noted today that if I give enough gas to cause it to start, then back off a little, then give a little more (past where it would have been an issue) it seems to work OK. Using this self-induced kind of surge - press, slight release, press some more, repeat - I can almost totally avoid the issue.