Retronics

I posted a video on YouTube to show the problem: Sorry about the shaky filming; I mounted the phone on a tripod between the front seats, but it wasn't very stable. Most of the time, you can see the temperature gauge in the bottom left corner. Does anyone know what causes the temperature drop, and how it can be solved? I bought this car from a Subaru dealer 4 years ago, and it has always been struggling to keep the right engine temperature. At winter, I have to drive uphill to get the engine up to working temperature, and when driving downhill, the temperature drops. I've described the problem to the dealer several times, and all they do is take the car for a drive, getting the engine up to working temp, and then telling me there is no problem. I also went to another Subaru workshop. They replaced the thermostat, but it didn't help. This video was recorded at -16°C. Prior to recording, the engine had been idling for 1/2 hour, and during this time the temperature rise was hardly visible on the gauge. Driving 5 km uphill got it up to working temperature. Then, during 7 km of mostly downhill, the temperature dropped significantly, and it got noticeably colder in the cabin. Lastly, 4 km uphill driving got it up to working temperature again. Like this, the temperature rises and falls in with the terrain. I took the same test run at -8°C, and the temperature drop was almost the same, and even at temperatures around 0 the temperature drop is noticeable. Changing the engine mode from Intelligent to Sport, to make it run at higher RPM slightly reduces the temperature drop problem, but it's still a problem, and I can't believe it's supposed to be like this. Any thoughts?