Battery 100%, sudden power off, SMC reset, Battery 0%

Quirky Macbook Pro

Sudden power off issues.

Battery will be fully charged, BAM!, complete power off. Reset SMC, battery now zero and will need 3 hours to recharge to 100%

Unit may work well for a week, no issues. Then back to square 1.

Resetting SMC will work kinda, but it's getting old.

New Battery, RAM checked and confirmed good. Unit cleaned for dust. Does not get hot, no high speed fans running. Unit functions w/o issue otherwise until it decides to throw a tantrum and start again.

Yes, I know it's a 2009 and past it's service life. I just like a challenge and hate to toss something just because it's quirky.

Thoughts?