Are you using a 3 spd OD with all of the original type electricals with it? I have heard of something similar happening with these setups. If I remmember correctly, it's related to the govenor, but I'm not entirely sure. Check a shop manual. Try disconnecting the overdrive and see if that cures it.
As I recall, the ignition is interupted when it goes into overdrive. If that part of the circuit isn't working correctly, that could be where the issue is.
