I have a 2011 VW Golf Mk6 Variant (European spec) with a 2.0 TDI Engine (CFHC engine code: with DPF but no AWD and no AdBlue), automatic gearbox (6-speed DSG), and 200.000 km on the clock (around 125.000 miles).
I have been facing a strange issue in the last year or so: the engine warms up normally (up to 90°C), and then the water temperature (as indicated by the temperature gauge on the dashboard) will randomly start to (slowly) drop until it reaches around 60°C (this happens while the car is in motion). It stays at 60°C for a short while, and then gradually starts to go back up to 90°C. It happens completely at random: sometimes it doesen't happen at all for hours, sometimes it happens multiple times during a single hour. The car drives normally otherwise (no power loss or anything like that).
Pulling over and restarting the engine does not help. The coolant level is fine (close to max). Moreover, there seems to be little to no connection with the outside temperature (the issue appears both in -10°C wheather during the winter as well as 30°C during summer) or engine RPM (it happens in both S gear - when the gearbox shifts at ~3000 RPM, and D gear - when it shifts at ~2000 RPM). However, the issue does tend to happen a bit more often when it's cold outside and/or when I'm driving at lower RPM (in D gear).
Futhermore, the Oil Temperature (as indicated by the on-board computer) also drops whenever the water temperature drops, but not as much (when the water is at 90°C, the oil is in the high 80s, and when the water temp drops to the mid 60s, the oil drops to the mid 70s, and comes back up once the water temp increases).
I'd also like to point out that issue happened roughly at the same time when I replaced the car's battery. I completely forgot to check the battery, and realized that it was due for a replacement only when it was too late: when it was unable to start the car and I was left stranded. The mechanic which replaced my battery with a new one assured me that my that car does not need any kind of coding after putting in a new battery (which is in line with what I've read on the Internet: VW only added mandatory coding after replacing the battery starting with the Golf Mk7, but mine is a Mk6).
I also have a VAG-COM (VCDS) clone which I've used to run an OBD scan, and there were no error codes in the Engine or Automatic Gearbox modules (the only modules which had errors were infotainment-related).
Any idea on what might be the cause of this strange temperature issue? I've read on forums and such that it may (or may not) be a bad thermostat or temperature sensor. I plan to take the car in for servicing at a VAG specialist soon, but I could use an opinion on what to expect (unfortunately, in my country it's not unheard of to be ripped off by mechanics unnecessarily replacing working parts).