amanico[JLC Moderator]
405123
It should last a bit less than 20 seconds. It needs a service.
Hard to say what can be wrong on your watch. But if you bought it at a on line auction, it may well not have been serviced for a while...
So, giving it to your watchmaker is the most prudent decision to avoid further bad surprises.
Very nice " Ebony " specimen!
Welcome here, too.
Best,
nicolas