Intermittent (shuts off randomly) power to one leg (hot) wire in sub panel

This is an apartment complex. The service panel has two rings, or meters, and 2 Murray 2-pole 50 amp breakers. The service panel is about 25 to 35 years old. Each 2-pole 50 amp breaker services a subpanel in each apartment. No issues with apartment A, but problems with apartment B.

I took a voltage reading off of the two legs from Southern California Edison (SCE) going into the main service 100 amp breaker for each of the apartments. All four read 117 volts. Two for each apartment.

I then went into the subpanel and took a reading directly from the two main wires coming from the service panel for apartment B, only one has 117 volts. Other is completely dead. Went back to the main 2-pole 50 amp breaker. Switched it on and off but still one “dead wire” at the subpanel. I then switched the 2-pole 50 amp breaker with apartment A’s 2-pole 50 amp breaker. Still had same issue with the apartment A breaker that I just hooked up to apartment B. Put original Apartment B breaker on apartment A and no issues with apartment A. Seems like both 100 amp main breakers are fine.

The “dead wire” in the subpanel randomly shuts off and on. Time frame is about every 30 minutes or so. This is even with the HVAC off so that is not the issue. The contacts for the main 2-pole 50 amp breaker are not scored but I don’t have access to see where the “dead wire” hooks up to the main 2-pole 50 amp breaker contact because it is right behind the electrical service meter.

So I’m thinking that the contact I can’t see may be loose or bad or could the SCE Smart Meter be “blocking” electricity to go to one leg?

1) Do I need to contact SCE to have them remove the meter?
I know how to replace the whole dual service panel if necessary but I’m not a licensed contractor.

2) Do I need to be a licensed contractor to replace the panel if necessary? This is in California.

Any suggestions on further diagnostics would be helpful.

Thank you,

Phil