dpb wrote in :

OK, I've had a continuing hassle develop w/ the ground in the old barn
over the last year or so.

Finally, about two months ago I replaced the ground rod w/ new and all
seemed well. As of about a week ago, the gremlin is back--there's
enough to light a couple 100W bulbs at not quite full intensity and
outlets measure full 125V but not enough current to power motors, etc.
Clearly it's the ground as all the 240V gear is fully functional.

No, very clearly it is *not* the ground. It's the neutral.

Ground and neutral are not the same thing -- and if you don't understand this, you have no
business monkeying around with this stuff.

It's _extremely_ hard to fathom a new rod can have gone south so quickly
and we've had sufficient rain that it certainly is the case of
excessively dry ground.

Yesterday I ran a jumper directly from the ground bar in the circuit box
to the ground and made no difference whatsoever in the symptoms.

Of course not. You don't have a grounding problem. You have a problem with the neutral.

It is _all_ 120V circuits, not just one so seems as though not possible
to be a failed breaker not passing current; but for the life of me I
can't figure out another common-mode cause...

I can. It's the neutral.

Anybody got any ideas or ever had such a symptom? I may end up calling
the pro on this one...

Sounds like a good idea. I think you're in over your head.

was out just last week to help find a broken
underground feeder to another of the outbuildings; too bad the symptom
hadn't reared it's head again then or woulda' had him take a look then.