unstable coolant temperature in a 200 20V; sound familiar to anyone?

[DeWitt Harrison]

The symptoms are that the coolant temps jump around between
a bit too cool and a bit too warm. This a a real fluctation seen
both on the dash temperature gage and on an independent
thermocouple probe clamped to a place next to the comination
temperature sender. In dash and independent meter always in
close agreement. In general, this 200 20V car runs cooler on
the highway and hotter around town. In the past, the temperature
had always been rock steady.

No repairs attempted to date have altered this situation in the
slightest. A new thermostat made zero difference (the orginal
tested good anyway). The system almost acts like there is a
hidden thermostat somewhere that has a mind of it's own.
Maybe trapped air in the system? But how would that be
possible? Ideas anyone?

Thanks,

DeWitt Harrison
'87 5kcstq