727 sometimes not going in reverse

I bought this vehicle and found that sometimes it will not go into reverse. It has a cable shifter on it and often, especially if at an incline, you cant get reverse to grab. I rebuilt the transmission and installed and it went into reverse more than it used to but still sometimes wouldn't! I then thought it had to be the valve body so I swapped the valve body from one I knew worked previously and it still acts funny sometimes. It will lock up like it is going in reverse but pushing the accelerator does nothing. However it doesn't roll forward like it's in neutral. Also it seems to mainly happen when it's at an incline. Ant suggestions?