Now the breaker itself could develop high resistance contacts....heating the
breaker internally and causing it to trip at a lower current than the rated
20 Amps. I suspect the breaker if the insulation on the wiring is in good
shape. Also check the lugs and wires as they attach to the back of the
breaker. If these are loose or corroded, the heated lugs can also warm the
breaker causing premature tripping.

I thought it was the breaker, but the new one does alo trip.