I have 2 of them. I of them has jumped out of 4th 3 times. it seems only when cold. I have about 400 miles on mine. the second one has never jumped out of gear. I think it might be a shifter adjustment. they have gotten very smooth with miles on them.
I think mine is the shifter adjustment, because when in 4th gear the shifter is slightly forward. I can feel that it does not engage far enough into 4th. the other one engages much farther. also when in 4th, the shifter is farther back. I have not re-adjusted it yet. I will post back when I do. mine were E bodys, so you have to totally remove all the shifter rods to install it. that's why I think I may be off......