How does one know when a new fuel pump (in tank) is needed?

1989 240 DL auto. Tries to shut off when gassed hard at crusing speed and during acceleration. Put in fuel injector cleaner and things got much better. My mechanic says it's the in tank fuel pump. Any other ideas what it might be?

Thanks, Hector

1. changed main fuel filter yesterday
2. new plugs
3. new air filter
4. everything okay with AMM
5. cleaned throttle body