I have a 2002 Ford Ranger with four wheel drive. When driving, the OD light begins to blink and the transmission shifts out of overdrive. After stopping at a stop sign or light, the transmission shifts hard out of first gear. When I turn the motor off and restart, it re sets and is fine for a bit, then the light starts to blink and the whole process starts over again.
I believe the flashing OD light indicates the automatic transmission has placed itself in “limp home” mode . . . in order to protect itself from mechanical damage
Best take it to a shop that has pro-level scanner, can interpret automatic transmission pids, retrieve any transmission-related fault codes and properly diagnose what is causing this situation
Since this seems to be easy to duplicate . . . according to your description . . . a good shop should be able to diagnose it in a reasonable amount of time.
In other words, do not go to any of the big name chain auto shops, such as aamco or Pep Boys
How many miles?
Which specific automatic transmission?