Hey Dan:

Let me tell you about my 84 240 withthe same problem. I don't know how similar the cars are, but I think you will find that the information is transferable.

I'm guessing you have checked the your fuse panel. I kept having to replace the fuse that the overdrive circuit is on on my 240. Each time the fuse blew, I experienced the same symptoms you are experiencing. Finally, I quit blowing fuses because my overdrive relay just burned-up. What I found was that the single wire running from my shifter (button), through the underside of the body, and into the overdrive solenoid had, through years of exposure to oil and heat, lost much of its insulation. The wire was grounding-out against the transmission case. I replaced the wire and have not had problems since.

You can make sure that overdrive is always on by simply providing 12v to the overdrive solenoid. You would just have to run a wire from your fuse box to the solenoid. Use the same circuit (fuse) that normally handles the overdrive and you can be sure it has enough capacity. If you do this, make sure you unplug the relay (in case it is damaged/shorted). All in all though, this is a pretty un-elegant solution - kind of like me replacing fuses until my relay melted :-)