M5OD-R1 usually pops out of 5th

[TheAbstracted]

So I bought a rebuilt M5OD-R1 for my '88 2.3, and it's a lot better than my old one - except that it refuses to stay in 5th, at least most of the time. Sometimes it will stay on the first try, but usually it'll pop out 3, 4, 5 times or more before it stays. I've been trying to research why it would do this, but every answer I've seen pop up seems to relate to it never staying versus usually not. I think maybe I might have messed up the shifter mechanism because it was really difficult to get the stick onto it and I was really banging on that sucker, but I really wouldn't know if that would have anything to do with it, transmissions are not my forte. I did see one answer saying the shift fork for 5th can be adjusted by a star nut on the outside of the transmission, but even though my old one has that and I could see what it was doing while I turned it, my new one just tightens and loosens that same nut instead of it adjusting anything. Don't know, I do have a warranty on this if all else fails but I'm really hoping this is something I can fix myself without too much trouble, as it was a royal PITA to get my old one out and this one in by myself, which is what would be happening again if I need to do that.