727 2nd gear slipping

67 Dart with a 727 behind a 360
previous owner says a Shift improver kit was installed unknown as to what kind.
she slips in 2nd.
shifting manually 1st is good tight no slip.
shifting manually from 1 to 2 slips.
shifting manually from 2 to D good tight firm shift.
shifting manually from D to 2 slips.
in D shifting automatically shifts from 1 to D and skips 2nd sometimes.
any thoughts or help pointing me in the right direction would be great.
thx