Hi,

you may want to check your clock calibration.
either against a neighbor cell or if installed as option the gps clock source.

clock calibration is a value stored on the device and can be overridden in config in case it drifted too far over time.
this is not uncommon since the crystals age with use and time.
a recalibration every few years should be enough in most cases.

the behavior of the phone sounds a lot like what happens if the phone gets confusing clock-references from the networks it can receive and at some point decides which one is 'right' and ignores whats not fitting that raster.

regards