Despite replacing battery MacBook Pro 2010 still randomly shuts down

I'm so frustrated.

I just dropped $100 on a MacBook Pro 15" Unibody (Mid 2009/Mid 2010)

Replacement Battery to replace my 2010 Macbook's original battery. It was shutting off randomly during use, whether it was connected to the charger or not.

THE SAME THING IS CONTINUING TO HAPPEN despite the replacement of the battery by an IT guy specifically trained to handle macs.

PLEASE any advice would be great. It'll happen as I'm working and I'll lose most of the stuff I have up. My macbook is still restarting directly after its random shut down, so I refuse to think it's completely dead.

Any help greatly appreciated.

best,

Grace