You have done everything mechanical that I can think of. The only reasons I have seen things like this were because a feed roller had become too old and hard that it couild not grip the paper. Is it possible the paper is too slick to feed well? In the old 4+ and 5n models the worn out upper feed roller would feed the paper out into the stacker tray too slowly. This would cause the next sheet to back up and get folded up or jam. On these you could see an inch wide curling across the top of the page where the lower roller was pushing the paper faster than the upper roll could pull it along. Then you knew it was time to replace the upper roller. Hope this helps!
Al