This is certainly not my area of specialty, but IIRC, the overdrive is an electronically actuated system that uses relays. From my ancient EE background, what you are describing could be a relay problem. Past that, I am afraid I can be of little help, other than to note that if it is in fact a relay that is going wonky, someone that knows the electronics on these transmissions might be able to point you at one or two of them that are the likely culprits and replacing them will be much cheaper than a transmission rebuild. I hope, anyway.
Please keep us informed as you learn more.
pax, smn