According to Ajax :

Chris, I think that you were very close to what we discovered was the
problem. After replacing the 60 amp double pole breaker with another
new breaker, it continued to trip.

Glad you found it. The issue here was that conditions were such that slight
moisture/movement variation provided intermittent conductivity (or arc)
between the nicked conductors, and at other times, there wasn't enough
carbon path to show up on the instruments.

Lesson: Never trust a plumber with anything sharper than a can of PVC
glue.

Heh. The last time I did a full house rewire (actually a major duplex
rewire, two systems), every time I came over for a wiring stint, the plumber
had managed to do something awful to the wiring I installed the previous
time. Like the time his 3" drill caught a wire, and pulled a _whole_
circuit, boxes and all, out of the wall.

At one point, about 50% of each day was spent fixing what the plumber
wrecked from the last time.

[He really was a damn good plumber. It was a huge and difficult job. He
apologized, I eventually forgave him ;-)]
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.