Interesting temp behaviour

[fruitcakesa]

Having replaced the rad, tstat, cap, all hoses but the bypass and new MB coolant in my 81, I figured the car would run at normal 80*-90* range.
And it does most of the time, but when I either put it in neutral and coast {I know, bad safety practice] or shut it off after a drive, the guage will rise towards 100*. Once I put it back in gear and the revs come up, the temp gauge drops quickly. Same if I restart after a shut down of say 10 minutes or less and begin to drive , the temp also goes back to normal.
Is this an unusual temp fluctuation?
Outside temp is in the 70* F range