My Tdi's heat gauge shows a different but stable temperature every day.Sometimes it's mid gauge, sometimes it parks between the red line and the peg. The engine doesn't boil and the waterpump, heat probe and thermostat (3rd one) are new. The thermostat opens fully in a pot of water about to boil. Anybody seen this before?

 

Yep - the sender is failing, change it next time you do a coolant change, they're a very cheap part