Interesting temp behaviour
 

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

that happened to me a bit too until i unscrewed the expansion tank cap to the first tab (not all the way off but still loose) and drove it around a bit. got the last of the air out and it stopped doing that.

That will be easy to check. Thanks

Seems to make sense to me...the engine needs to be cooled and the water isn't flowing enough until you get the revs up. WHen I run my car for a while, park, then start again within 5-10 min, the gauge reads above 100 but quickly goes down as the coolant circulates.

When the engine is turned off the engine goes into hot soak. It will rise in temp for a bit and this is normal.