I had a 15/20 breaker go bad. Actually only the 20 amp side went bad and would trip several minutes after being reset even though the load on it was below the rating. Replaced the breaker and it never tripped again. Inexpensive and easy to replace to see if that is your problem.
If it still trips then I would guess you have developed a short somewhere.
Oops.
I overlooked that you had already replace it. Then I would have to guess a short.