Well on an 83 Town car it shouldn't even have the option to use a scanner. That should have an EEC III engine control system on it. A scanner wasn't used until Ford switched to an EEC IV system. It's been a while since iv'e tested one but you had to pull a vacuum on the MAP sensor three consecutive times to enter the test mode than count the light flashing , or something like that.
This might be a wild shot but I have came across a few of them that had similar concerns and found it to be a faulty Fuel Injector Driver located on the PCM. I noticed that usually one Inector stops working all together making it very sluggish and has poor idle quality. See if the injectors are both spraying after it starts acting up. If this is what happens then I would replace this, it unbolts from the main Powertrain Control Module. The other thing that might make it run rich is the MAP sensor, they are a common problem too in those earlier years of the evil EEC III era Powertrain Controls.
Hope the info helps you out.
Luke