Final update: After replacing the temperature sensor and thermostat on my 300 TDI, I discovered the 3-way splitter on the overflow line was misrouted, sending the main coolant flow to the expansion tank instead of the radiator. Correcting the...
Installed a new sensor and after 20 minutes the gauge does the same thing - goes to 3/4. Strangely enough, the gauge dips slightly when i press on the brakes and if I turn off the engine and restart it goes back down to below 1/2. I am now...