First time poster - any input appreciated.

I have a 2000 A6 2.7T with 170,000 miles that has developed an overheating problem.

It warms up normally and runs for awhile right in the middle of the water temp gauge then starts to move up to 2/3 to 3/4 on the gauge. Its always run right in the middle of the gauge and I had the water pump done back at 125,000 and am pretty sure the thermostat was done at that point as well.

Any thoughts on where to start? My first thought was thermostat but could it also be a sending switch? Easy way to check if its thermostat? The coolant reservoir cap seems to be starting to deteriorate - if theres a small pressure leak at the cap there could that cause overheating?

Thanks for any input!