For the OP, sorry but none of us will be able to answer your real question about the root cause of the variance and whether you need a service now or not. Several people, myself included have suggest that you wear the watch and enjoy it. You can then see how it performs over time and see if it runs consistently. As someone who has both newer and older watches, I would be comfortable with that approach, as there is an endless supply of parts for your watch. There is nothing that will wear, that an authorized repair person cannot source, and at 3 years of age, your watch is likely fine. The only alternative is to have it disassembled, cleaned and inspected and reassembled. This is an expensive solution to confirm the condition if you are the one paying for the service. Even if a service centre checks the regulation for you, this is no guarantee that everything is perfect inside your watch.
I recently bought a vintage zenith that was advertised as serviced and ran inconsistently. In that case, I wore it for a couple of weeks to see if it evened out and it did not. The auto wind did not keep it fully wound and it varied in accuracy from day to day. In this situation, parts are hard to chase down, so I got it in for service, even though it was advertised as a serviced watch.
