On 12/2/2015 2:58 PM, dpb wrote:

The fact that the different strings don't appear to make any
difference to the result *suggests* it's not in the "load".

I just had basically agreed with that assessment...as said, hadn't seen
that diagnostic result previously.

It could still be a problem with the strings of lights; they have all
seen roughly the same sort of use, over the years. Something that happens
to one lamp in one string could also have happened to another lamp in
another string, etc. With 25 lamps in each string -- plus a plug on
one end and an outlet on the other, that's ~80 places a problem could
infest!

This leaves outlets, wiring and extension cord.

The fact that lights plugged into extension cord doesn't cause a
problem on another branch circuit suggests the cord MIGHT not be a
problem (it could still be "leaky" and the non-GFCI doesn't notice
that).

Precisely; since you later say you have other extension cords
I suggest swapping it out on the same breaker next before doing anything more
involved.

Too late to do anything more, today. I'll just turn the lights on
and deal with it, again, tomorrow (if I have spare time).

I can remove receptacles from the wiring (a bit of a hassle to remove
the "outdoor" covers, remove the receptacle, then remove the two
conductors -- I can leave the safety ground as long as I ensure the
receptacle is "dangling freely")

Isolating portions of the wiring gets to be a bit more involved;
instead of "daisy chaining" through the second set of contacts on the
receptacles, I tie the upstream and downstream wiring together with
wire nuts and run a pigtail off to the receptacle. This ensures the
receptacle is not "in series" with the downstream devices.
....

Any of that starts to seem pretty far-fetched to me albeit almost anything is
possible. OH, just a really far-out thought--you don't have any of the "crazy
ants" there, do you? They can cause all kinds of really bizzaro
thingos/symptoms if they've found a receptacle box and set up shop...

We have "leaf cutter wasps (bees?)" that fill little holes with rolled up
pieces of leaves that they use to make their nests.

Other wasps (?) look for little holes to fill with *mud* nests.

And, of course, various little spiders, etc.

It seems like the size of the safety ground hole in an outlet is
what they target. E.g., I have found that if I leave 1/4" irrigation
tuning on the table in the back porch, when I later go to use it, one
(or both!) ends will be neatly plugged with mud.

That's why I think removing the receptacles (starting with those that
aren't currently powering loads) is a quick first step. Easier than
messing around in the panel (I can turn off the breaker and work
on the receptacles without fear of catching a jolt).