According to gartulan :
Would it be worthwhile to switch to move a breaker to a free slot on
the other side of the panel...say on the bathroom circuit which is
brand new and we know 100% goes from the upstairs to the panel with
nothing in between----to see if we have the same poor current
conditions?

When people say "side" of the panel, they're usually _not_
referring to whether it's the left or right side. They're
referring to whether it's one hot or the other. Hint:
they mix (usually alternating) which breaker is on which
side of the main. So simply flipping the breaker from one
physical side of the panel to the other may or may not
prove anything.

If you have more than one circuit behaving badly, it's
in the panel or upstream of it (the service entrance).
Moving breakers back and forth won't give you any additional
information that you can use in a useful fashion.

Given the skill level that you seem to have, you have
probably already gone as far as you can go safely.

At this point you should either call the electrical
utility or an electrician.

If it's in the panel, it'll just take a few minutes to fix.

Sit down and think about your situation:

1) if some circuits are dead and/or weak, it's the panel
or the main feed (electrician or utility).
2) if some lighting circuits are _also_ brightening,
it's a loose neutral, and you should try the utility
_first_.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.