I still suspect the breakers. I have seen the same thing with GE breakers that were unsually shiny.

"The house panel has two 20 amp circuits (separate phases, linked together) that feed the garage sub panel."

If these are the feeders to the sub-panel you have 240 V if they are on separate legs, but only 20 amps of available current on each leg for 120 V use, or a single 20 amp 240 V load.

If the feed is long or the branch circuit long the starting surge can be extended for a 120 V motor.
This can trip the breaker since the surge may be well above 20 amps.

Household breakers are thermal-magnetic.
The thermal portion provides a 'slow blow' for short overloads, while the magnetic trip provides a very fast reaction to short circuit (300%+ of rated current).

Long conductors create more voltage drop, and this can become critical under starting current (2-3x running current).

The voltage drop increases the starting current and the starting time and can exceed the 'slow blow' built into the breaker.

How long are the feeder wires from the main to the sub panel? What size are the feeder wires? I am suspecting a voltage drop issue.

Thanks for answering the 220 question although the wires going out to the garage probably won't support it.

The feeder wires only appear to be either 10 or 12 gauge. I'll need to pull the panel cover off again to see for certain. The line run from main panel to sub panel is only about 40 feet, going underground from the basement and coming up into the detached garage.

An added twist is that prior to my ownership, the dirt basement craw-space was the neighborhood cat toilet. A junction box for the garage circuit is laying on the dirt floor and wires go from there underground into the dirt and down and out. The box is eroded from acid in the cat pee and in some places, the armored cable is completely eroded through and only the wires are left exposed in their plastic wrapper. I haven't had the opportunity to tackle that one yet nor have I seen how bad things are inside the box. It may be playing a role with the current loss theory.

Fixing that whole mess is another problem in of itself as I don't have any room to play with extra wire. The junction box is already on the ground in the dirt. And, they seem to have just run armored cable into the ground but there is conduit coming up in the garage so it must meet up with it somewhere. A mess, but fairly typical for this 1896 home.

"Thanks for answering the 220 question although the wires going out to the garage probably won't support it."

You either have two legs of the single phase 120/240 V service and have 240 volts or you do not.

Wire size does not determine voltage, it only determines ampacity.

Even #30 wire can carry 240 volts, it just cannot support more than about 5 amps.

Thanks. In my comment above, I meant that the wires probably won't support the amperage load that I would like out there... 40 amps. 220 itself should be fine as I know I am pulling in both phases.

any update on the tripping?

No, but I am going to try a replacement breaker just to see. I'll follow-up after I try it.