Temp gauge fluctuation or wavering can be all sorts of things, but usually it is a sign the connections on the actual gauge pod are oxidized or corroded, happens alot what a car sits for a time. ( 2 years)
Can easily be fixed, but you wont like how....pull the cluster, open it up and remove the needles and take off the faces and remove the actual motor that moves the needle. Lots to do to get to the motor not hard, but not easy or for the faint of heart. After it is off the circuit board all you do is clean the contacts wil a wire brush and reinstall. A pegged or maxed out gauge that has good wire harness and sender is a typical candidate for bad or corroded connections. IT grounds itself and that makes it go to the max position. Also a full grounded sender or a bad sender can do the same thing, you said yours is new so it is ok and you said the gauge reads 95 to 100 so the wiring should be fine. the waver comes in when the gauge motor for the needle gets small intermittent contact to the circuit board and it cause the needle to jump or peg at max. i just fixed one that was pegged and all it was was oxidized pin contacts on the needle motor. Cleaned them and reinstalled and it was good.
If you want to test your gauge I can explan how. You will need to go to radio shack or any place you can buy electronic resistors though. One small resistor and a length of wire and you can test the temp gauge to see if it is reading correctly.

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red