Just to add to the confusion and maybe an answer.
When I was still in the business, we bought 25 FPE breakers in a pack one time. (Not that I was a big FPE fan) but we had all of them with issues of what we called "weak" performance because they would trip for no apparent reason. We had one that would trip when a toaster was used on the circuit.
We set up a separate FPE test panel in the shop with some big honkin' switchable resisters and a couple of ammeters and found out that there were two reasons the breakers would trip at low loads. The big reason was a bad batch of breakers had the temperature limits for trip current heating set too low when they were manufactured and the other, although a minor issue, was slightly misaligned connection lugs which made a little bit of temperature rise in the breaker.
I think those breakers were manufactured in 2000 or maybe early 2001. I can't remember the date codes exactly but it was somewhere around that time period. I know they were of Chinese origin. Maybe you had some from that same batch.
We replaced all the "weak" breakers with new ones and the problems went away.
Hope that helps you out.
