Thanks again guys. Doing some more research, it looks like I've got to pull the codes but I think I'll be putting in some solenoids:
Overdrive light flashing on and off. - YotaTech Forums
leads to
Support Groups for Toyotaholics - A340 Transmission issues and FAQs
most likely leads to
Solenoid kit, A340, 2 Shift solenoids & 1 Lock-up | Switches & solenoids | Bulkpart transmission parts
Could be a relatively inexpensive fix. Time to get the volt meter out.