Cuckoo clock sounds 13 times sometimes?

[yahwhoooon]

I just bought a German Cuckoo clock and it works fine but every day at a different hour it will sound off 13 times, could be at 5 oo'clock, 2oclock etc etc then it is fine for the rest of the day, most of the time its ok but then all of a sudden at at a different time it will go 13 times instead of the correct time, what could cause that?