Please, your candid advice is needed

Hello peeps, I don't really understand what could be wrong with my Nokia E5 battery. Sometimes, when I try to browse, make or receive call, it goes off but it doesn't go off when I playing music . Please, where in Port Harcourt or Lagos can I buy orginal battery for Nokia E5? Any other idea I can approach to solve the battery issue is welcome. Thanks for your anticipated response.

go to slot and get an original nokia battery..

@Oyi; I appreciate your response but I want to ask, is there slot is Port Harcourt?

Slot is in Portharcourt, along Aba road- opposite water lines junction