Flying with default ATC giving vectors, the aircraft (Autopilot, FMC or whatever) dials to another frequency after 30 to 60 secs. all the time, so one looses connection to ATC.
Have an example here:
https://youtu.be/L6f8uYyTE8s

Very frustrating error since it makes flying the 737 very demanding, always setting back the frequency to get instructions.