Mine was misbehaving in a similar manner............pull it into low and it wanted to stall.
Like it was trying to move in 1st and 2nd at the same time , so it just wouldn't move.
Still puzzling it out , but it seems to be something with the shifter cable adjustment.
One direction and it shifts fine , but will not go all the way into park.
The other and it goes in park just fine but often acts like it is not all the way into first.
Cheetah shifter with a longer 5" cable and Turbo Action valve body.
I switched from a Griner RMVB to the T/A and changed the shifter gate to a forward pattern because I never did warm up to the reverse pattern.
And it has been being difficult since.