Alright, so first off, I've got a 99' A6 Quattro 2.8. The issue is that periodically, when I start it up and it goes through its "warm-up" mode and the engine temp slowly but surely rises to normal, sometimes when I stop accelerating, the temperature will drop right back down to "C."

It only started happening recently. I'm not sure if it has anything to do with the cooler weather or what, but it's definitely never happened before. Any ideas?

 

Search for coolant temperature sensor (CTS). 15 min fix.