I've read a few threads where people had the same problem as myself. My car only has 63k miles and was not raced. I tried adjusting the cable and go from a forward gear to reverse and then sometimes I sit in neutral then go to reverse but it'll pop out as I let the clutch out. I've had luck with holding it in reverse but then it'll pop out and grind bad. I can feel it go into reverse though, I don't know what's wrong, it's not normal. Any ideas on what to look for?

 

mine does that too, but its due for a new transmission - hopefully thats not your case

 

Reverse is a bitch on most Mustangs, it definitely is on mine.