I'm either completely lost or overthinking an issue I've got going on.

I've got a 2-1 car that is stopping suddenly when it passes the 136n (ETS) magnet. The car throws 101, 126, and 210x (whichever one is fault not present long enough to track.) car stops suddenly, then levels in a few seconds later and goes back in service. Is this an ETS stop or something else? I assumed it was going too fast and thought about doing a decel and etsl learn, but have no documentation on how to do that.
Another mechanic suggested changing the tach which has been done.

Thanks, and any help is appreciated.