1960 Merc-o-matic Question

[komet kustom]

OK,

I drive my '60 quite regularly now here in Chicago. On an occasion when coming to a "near stop", like a yield, the trans slips into a neutral position before shifting into 1st. Does not do it all of the time. She shifts into drive well and solidly when selecting from park. Reverse selects fine too. Up shifts are solid and it never hunts for second under any driving circumstances.
What should I check? Could it be the linkage or a vacuum line? Modulator? Fluid was drained, including the torque converter late last year and replaced. This car only has 34K miles on it but sat dormant from 1972 till last year in a field. No fluid leaks.

Any ideas are greatly appreciated as do not want to do damage internally!

Thanks ahead of time,
Duane