Make sure the spring tension is correct (5.2") on both sides of the chain and that the tensioner hasn't bottomed out on the slot on the side of the feederhouse. If it's bottomed out you will have to remove a half link. Make sure doublenuts are tight on springs. Check for any bent bars on chain; a bent bar will force the chain to move left_right as it travels around sprocket.
I solved chain jumping on my R60 by loosening all bar mounting nuts, rolling the chain a few revolutions by hand to center it, and retightening the nuts. Good luck! Chain jumping can make a guy want to hire a custom cutter.
There is such a thing as strippers for both front and rear chain. For the front chain there are pointed chunks of iron that would fasten to bottom of square tube just in front of sprockets. They help keep any stray cobs from getting inbetween chain and sprocket. I would rather not use them and may well not need them if you run chain on fast. Fast is front right feeder belt on the large front pulley and smaller rear pulley. Maybe you could benifit from running rear feed tention chain on grain rear position if not allready. That may keep cobs clear from transition area between chains. Same thing with strippers and feed chain speed applies to back chain. Quick way to jump chain back would be insert 1_2" bolt inbetween chain and sprocket just far enough to hold link as you turn feeder over by hand. Insert bolt where you can notice chain not fitting down into sprocket or where you notice chain looser. Take care