Hi Boeing,
As a rule of thumb when a chain on my ride on or reel mower jumps off by its self ie: without any reasonable explanation I replace it.
If the chain is stretched it will continue to distort the sprockets and cause wear, so it�s safer to replace it. Also look at the drive sprocket, if it has thin pointy teeth or missing ones replace it too. A new chain will soon be thrashed by a worn sprocket. As far as I know the bigger sprockets don�t wear as much because they always have more surface contact, and share the load more evenly.
Regards Jay
