Tim Quinn,
It would have been better to start a new thread so answers & comments on your &
zoegrant's questions don't get mixed up. It's something you can still do and I recommend it.
But since I already typed this comment before I noticed where I was...
Look very closely at the overdrive wiring diagram. There is a circuit in the oem wiring that momentarily shorts out the ignition points in the distributor so the engine hesitates, taking the load off the gears so the OD can safely shift. This may be what you have encountered.
It may be something before the 'white with red stripe' wire between the kickdown switch and ign coil on this diagram. (not the wire of the same color to the oil pressure switch)
Disclaimer: I haven't yet fixed the broken OD wiring in the '57 I have, so I'm still guessing too.
Wiring diagram link...
http://www.oldcarmanualproject.com/t...re5765-195.jpg
.