Prefacing this reply with I have no idea about that model year, my 89 uses a 5v regulator from the gauges and its technology dates from 1937.
Mine looks like this, and I see the same part back into mopars of the mid 60's
When my gauges started acting funny, I replaced this gizmo, with a more modern version.https://www.amazon.com/DROK-Waterpr...mp;psc=1&refRID=P1NZ1K3KE5PYWKWQJNW5
This one's voltage is adjustable and has been powering my gauges for about 3 years IIRC.
Also check that the ground wire for the instrument cluster is actually well grounded. Mine is grounded to the dashboard, and the dashboard's mounting screws might not have very good contact with The body/frame anymore. And of course the main battery to frame ground is usually an issue at some point too.
the pins' on the clusters primitive circuit board are also known to develop fractured solder joints which can cause your wacked out gauges. Reflowing the solder on them is pretty simple and could be effective even if it passes a visual inspection of them.
No idea about the speedo. Reed seems to have a good grasp on their functioning internal doodads