[Jeff Hail]

If your not comfortable or experienced working inside a service panel call an electrician.

Start with replacing the breaker. If its tripping with elevated temperature good chance its aging out. The cycling of the pump motor can wear a breaker over time. How does the throw feel and the engagement of the breaker compared to the one for the booster below it? Same feel, a bit looser, does the engagement feel firm when switching it from off to on?

Does the ground fault test and reset function work properly?

The pump motor has a thermal protector and the pump would cycle on and off it were the motor getting hot (it sounds ok).

Any sizzling sounds in the breaker switching from off to on with the pump set to run?
Check that smudge, wipe it with your finger and see if it has a odor to it such as english car wiring loom. Put your nose up to the breaker and see if you detect anything?

Visually check the hot bus bars that the breaker straddles on in the panel when you remove the cover, look for arcing of the contacts.