Chev TH400 Trans Shift Problem

[Ol'Jim]

My 71 Monte Carlo has a cable actuated B&M Shifter in it.
Last week, it started acting like the cable was out of adjustment.
I could not get it adjusted to where it would shift properly going up or down, (and adjustment doesn't normaly change once it is set correctly). This setup has been in the car for over 10 years, although I replaced the cable a couple years ago because exhaust heat damaged it.
Changed cable yesterday, problem sill exists. From Neutral, to get into Reverse, I have to shift past R, almost into Park. But going the other way, from N into D, I have to go down to where 2nd gear is. So obviously it is not cable adjustment, because I have to go 2 shifts forward to move 1 gear up, and 2 shifts back to go 1 gear down. Seems to be getting worse, now I cant get into Park, or low. Acts like there is a lot of slack in cable, but there is none. Could the selector shaft inside the valve body be broken, or? Please help, any and all advice will be greatly appreciated! -Jim.