Probably a fault inside the movement. I had a similar issue with one of my clocks. In the end, on the basis of nothing to lose, I took the movement apart and removed the PCB circuit board, noting there was a lot of chafing where the battery contacts pressed (amazingly not soldered on as the metal chosen didn't "take"solder).
I just built up the PCB tracks a little where the contacts pressed and put everything back together. It's been going well for weeks now.
The alternative would have been to replace the movement with one from eBay, Amazon or a clock parts supplier. I might even have gone for a radio or WiFi controlled one.
Or just buy a new clock with a guarantee.