There are two test

The first is to verify if it is the original .. if so, replace it-regardless.
you will have to pull it to see the part # on the side.

The second test requires a highly technical approach... this is the
Tappity-Tap test..
Next time it does not start, you lightly tap it with a screw-driver handle and retry the start proceedure..

oh, I almost forgot..
This tapping tecnique is best done to Standard Benz/Joplin metering..

"Lord ... won't-cha-buy-me...aaaa ..Merc..say......days- benz.... oh Lord ..."

The tempo would be a slow , Blues feel..