Blackberry turns off while receiving calls and sending text messsages

I Have an old Blackberry Bold 9900 which turns off while calling or texting.

While Charging - Doesn't Turn off by calling or texting
Unplugging charger "Battery's fully charged" - Doesn't Turn off by calling or texting
After half an hour "Battery is around 80%" - Turns off by calling and texting
After 3 Hours "Battery 40-50%" - Randomly Turns on and off on the boot loop

Do i need to get a new battery or is that a hardware issue any clue guys ؟