Hi, when it "stops", does it shows the big blue pause/triangle icon on the screen? If so, then it sounds like Auto Pause is enabled and is activating unexpectedly. I have that on rare occasion, and I think it's just an occasional dropout from my wheel speed sensor. (Locking up the brakes on gravel or when airborne will also do it haha.) It only lasts a second or so. If it's happening frequently tho, then maybe there's an issue with your wheel speed sensor (low battery?).
If instead it shows the big red square stop icon and then jumps to the screen with options to save/resume/delete the activity, then that's a hard stop that should only occur by pressing the stop button. In that case it's possible the button switch might be stuck/failing.
