I have this symptom myself, on mine it's an inaccurate sender, I don't worry much about it (because I know it's off)! BTW, 90 degrees C. isn't that high for a 195 degrees F thermostat-could just be slow to open, some members have also reported trouble with worn down/corroded water pump impellers that cut down on the flow somewhat.

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1982 300DT 190K (Diesel Purge + synthetic oil=smoothness at last!!!)
2004 Ford E-350 6.0 L PSD 227K
2006 Dodge Ram 3500 SRW HO Cummins 4X4 48RE 42K (brute force tow vehicle)
2005 Scion xB wife's rolling pop can
1993 GMC Sierra C3500 6.2 142K