inconsistent engine temps in 2000 lxi D60 . when Hydronic heat is off or on

[luckyron1]

We're holed up in a Kansas city Flying J enroute to Branson, and then
Texas for the winter. While underway I've noticed some inconsistent
engine temps. We had the hydronic heat on until we got out of the
snow belt and the D60 engine temperature was consistently normal at
about 195 F. We turned off the Hydronic heat this morning and I soon
noted engine temps of 210 to 220 F. No leaks anywhere and the coolant
level is right where it should be. I put the hydronic heat back on and
the engine temp went right back to normal. Tested it a couple more
times and the same thing happened. Have we got a sticky thermostat or
what? Anyone know of a good Bluebird/D60 service facility in the
Kansas City area?