With out knowing what modifications have been made to engine?
What intake/exhaust are you running?
Have you checked for vacuum leaks?
If you are running the Model A manifolds and compression is too low, maybe the engine is having a hard time pulling fuel in at lower RPMs. I ran an early aluminum intake (Ford) for a while, the engine even with good compression (in the 50 LBs range) was not happy at lower RPMs with the extra volume that the intake had. Switched back to iron type and it was much better.
Going back to what I suggested before; check your compression and adding to that, vacuum leak check and maybe bleed down test. If the manifold has a port might also do vacuum gauge test.
https://www.fordbarn.com/forum/showthread.php?t=278006