A couple of years ago my 84 did the same thing.  The thermostat would open
and close fine when boiled.  The problem turned out to be that the flat
rubber ring that occludes all those little holes around the outer perimeter
had disintegrated allowing coolant to flow through the closed thermostat.

Charlie P.

> Yup, you read it correctly.  Lisa's 83.5 is underheating.  It has a
> fresh radiator, a fresh tested thermostat and a fresh couple of
> gallons of Dex-cool and distilled water.  Sitting in the driveway
> idling, I can get the temp gauge up to the led after about a half hour
> of running.  On the road, it immediately drops right back into the box
> at the left side of the gauge.  Most of the time it's clear down on
> the peg.