81 300D N/A
new thermostat (1 week ago)
radiator flush w/ MB Citrus (1 week ago)
MB coolant mixed 50-50

Today as I drove to work (without A/C), all seemed well, then the temp quickly went from 180F to 200F. I turned on the defroster and the temp went back to its previous level. I turned off the defroster and the temp stayed at the 180 level. What is happening here? I am concerned at the quick temp rise before I turned on the defroster and that something is stuck and not letting coolant in or something else.
Any clues?

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81 300D N/A 130K miles (Nelly)
87 300D Turbo 80K miles (The Scalded Dog...because it runs like a scalded dog)
95 e320 Wagon 104K
2003 Sprinter 47K miles.
1999 Tandem Bicycle
One beautiful low miles wife (who likes diesels) (that's my wife holding the sign)

My son and I fixing Nelly's odometer