2000 A6 2.7T overheating

[kf21857 at comcast.net]

I was on the freeway this afternoon when my 2000 A6 2.7T began overheating rather abruptly.  I pulled off and let it cool for a few minutes until the temp gauge dropped to a reasonable level, then made it to my destination, which wasn't far.  I had to stop several times for the trip back home.  The coolant level was down a couple of quarts but not extremely low.  No visible leaks and the coolant level isn't dropping.  The cooling fan is running on low speed but the engine compartment doesn't really feel that hot.  The car has been running fine even on hot days so something failed suddenly.  Bad temp sensor?  Stuck thermostat?  Internal failure in the water pump?  I am thinking of temporarily replacing one of the coolant temp sensors with a mechanical temp gauge before I start tearing things apart.  Any other suggestions?
thanks,
Dave